Let's examine the math for how this hypothetical flying bowman is driving off the Tarrasque.
For the same of argument, I'm going to assume that it's possible despite the Tarrasque being immune to fear, but it has to be brought down to half HP, which requires 336 points of damage.
Say the hypothetical archer has a +1 longbow with +7 to hit for 1d8+4 damage, average of 9. In order to hit the Tarrasque's 25 AC, they need to roll an 18 or better, so a 15% chance of hitting, and I'm going to ignore the effects of crits by assuming that they average out. That requires 37.4 successful hits- call it 37. And with a 15% accuracy rating, that works out to about 249 attacks. Just how many arrows is your archer carrying? Because that's 25 minutes worth of shooting at the thing assuming you never have to go pick up more ammo.
On top of everything else, firing a bow repeatedly is fairly strenuous activity. As a GM, I'd be inclined to start calling for constitution saving throws to avoid exhaustion after a few minutes. Less if the character were flying under their own power instead of by using magic.
Overall, the hypothetical situation of the lone/small group of flying archers stopping the Tarrasque by themselves is honestly not realistic.
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Find your own truth, choose your enemies carefully, and never deal with a dragon.
"Canon" is what's factual to D&D lore. "Cannon" is what you're going to be shot with if you keep getting the word wrong.
Charger. If the Tarrasque moves at least 20' towards a target and then hits it with a Horns attack on the same turn, the target takes an additional 22 (4d10) piercing damage and must make a Strength save (DC 27) or be knocked prone.
Damage Threshold. If damage to the Tarrasque from a single character or other effect is less than 30 in a single action, the Tarrasque takes no damage.
Elder of Annihilation. The Tarrasque's damage cannot be reduced by resistance or immunity. If the Tarrasque attacks a force object, that object is destroyed.
Juggernaut. No form of terrain costs the Tarrasque extra movement to pass through. No effect can reduce the Tarrasque's speed. The Tarrasque may use its burrow speed to move through solid objects as long as they are no tougher than stone.
Legendary Recovery. If the Tarrasque takes no action on its turn, or has its action compelled by an external effect, it reduces the duration of any number of effects on it by one round and may make a new save against any remaining effects, even if that effect does not normally allow repeated saves. It then heals by 80 hp. The Tarrasque may also trigger this at the end of another creature's turn by spending 2 legendary actions.
Legendary Regeneration. If the Tarrasque starts its turn with unspent legendary actions, it heals by 40 hp per unspent action.
Reflective Carapace. Any time the Tarrasque is targeted by a magic missile spell, a line spell, or a spell that requires a ranged attack roll, roll a d6. On a 1 to 5, the tarrasque is unaffected. On a 6, the Tarrasque is unaffected, and the effect is reflected back at the caster as though it originated from the Tarrasque, turning the caster into the target.
Save Substitution. The Tarrasque may use its its proficiency bonus in place of any save.
Siege Monster. The Tarrasque deals double damage to objects and structures.
Swath of Destruction. If the Tarrasque moves within 5' of a creature or object, that object takes 10 (3d6) damage, but no more than once per turn. Creatures may make a Dexterity save (DC 27) to avoid this damage.
Unkillable. The Tarrasque cannot die.
Arena of Earth (Recharges after ten minutes). If the Tarrasque would be reduced to 0 hit points, its current hit point total instead resets to 600, and for the next ten minutes the earth shakes, as per Earthquake (save DC 22), though it does not count as a spell and does not require concentration. This area is surrounded by a dome of stone 5' thick (treat as a wall of stone: AC 15, 30 hp per inch). Two fissures (as the lair action below) appear within the area. For the duration of the quake, the Tarrasque is considered to be within its lair and may use lair actions. If the Tarrasque is reduced to zero hit points during the quake, a fissure appears under it, it falls in, and is entombed, as if affected by the burial effect of the imprisonment spell. Award a party an additional 155,000 XP (310,000 XP total) for defeating the Tarrasque within the arena of earth.
Actions
Multiattack. The Tarrasque can use its Frightful Presence, uses one of Bite, Swallow, and Horns, and makes two Claw attacks.
Bite. Melee Weapon Attack:+19 to hit, reach 10 ft., one target. Hit: 36 (4d12 + 10) piercing damage. If the target is a creature, it is grappled (escape DC 27). Until this grapple ends, the target is restrained, and the Tarrasque can’t bite another target.
Claw. Melee Weapon Attack:+19 to hit, reach 15ft., one target. Hit: 28 (4d8 + 10) slashing damage.
Horns. Melee Weapon Attack:+19 to hit, reach 10ft., one target. Hit: 32 (4d10 + 10) piercing damage.
Roar (Recharge 5-6). All other creatures with 120' of the Tarrasque must make a Constitution save (DC 27) or suffer 22 (4d10) thunder damage and be stunned until the end of the Tarrasque's next turn. Creatures that succeed at the save take half damage and are not stunned.
Tail. Melee Weapon Attack:+19 to hit, reach 20ft., one target. Hit: 24 (4d6 + 10) bludgeoning damage. If the target is a creature, it must succeed on a DC 27 Strength saving throw or be shoved back up to 20' and knocked prone.
Frightful Presence. Each creature of the Tarrasque’s choice within 120 feet of it and aware of it must succeed on a DC 22 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, with disadvantage if the Tarrasque is within line of sight, ending the effect on itself on a success. If a creature’s saving throw is successful or the effect ends for it, the creature is immune to the Tarrasque’s Frightful Presence for the next 24 hours.
Swallow. One Large or smaller creature the Tarrasque is grappling must make a Strength save (DC 27) or be swallowed, ending the grapple. While swallowed, the creature is blinded and restrained, it has total cover against attacks and other effects outside the Tarrasque, and it takes 56 (16d6) acid damage at the start of each of the Tarrasque’s turns.
If the Tarrasque takes 60 damage or more on a single turn from a creature inside it, the Tarrasque must succeed on a DC 20 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of the Tarrasque. If the Tarrasque is reduced to 0 hp, all swallowed creatures are automatically regurgitated.
Legendary Actions
The Tarrasque can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. The tarrasque regains spent legendary actions at the start of its turn.
Charge (costs 2 Actions). The Tarrasque moves up to its speed and uses Horns.
Chomp (Costs 2 Actions). The Tarrasque uses Bite or Swallow.
Earthbind. One creature within 300' that the Tarrasque can see must make a Constitution save (DC 27) or lose the ability to fly for one minute, possibly causing it to fall. An affected creature may repeat the save at the end of each of its turn, ending the effect on a success.
Move. The Tarrasque moves up to half its speed, and gains 40 temporary hit points.
Tail Swipe. The Tarrasque makes a tail attack.
Lair and Lair Actions
On initiative count 20 (losing initiative ties), the Tarrasque takes one of the following actions
Open Fissure: a fissure 10' wide, 60' deep, and extending the length of the arena appears. Creatures in the area of the fissure when it opens must make a Dexterity save (DC 27) or fall in, taking 21 (6d6) falling damage. Structures in the area of the fissure are destroyed.
Close Fissure: One fissure within the arena closes. Any creature in the fissure at the time it closes suffers 21 (6d6) bludgeoning damage and must make a Dexterity save (DC 27) or be restrained; whether or not it is restrained, it lacks line of sight on any other creature until it exits the fissure. A restrained creature may spend its action to make an acrobatics or athletics check (DC 20); a creature that is not restrained may climb towards the surface but treats the fissure as difficult terrain in addition to any climbing penalties.
Eruption: a geyser of lava erupts from the ground, in a cylinder 10' wide and extending 20' above the ground; if the eruption occurs over a fissure, it also extends to the bottom. Any creature in the area must make a Dexterity save (DC 27) or suffer 21 (6d6) fire damage; on a successful save, damage is halved. The geyser lasts until the next lair action; any creature that ends its the area must make the same save.
Haven't really crunched the CR numbers so it might be off. Its raw damage output is I think lower than vanilla.
Once again let me reiterate that by the time you can manage to plink your way through most of 700 HP on an AC 25 monster, it’s had enough time to walk up to castle, eat the king, and walk back out. It’s weakest attack does almost 50 damage on average to structures with enough to hit bonus that only a nat 1 will keep it from damaging the target.
Let me reiterate that a segment of a structure is 10'x10'x10', so it's going to need to destroy two segments. And that stone walls have 30 hp per inch, so a 5' thick wall has 1800 hit points. Meaning it's going to take on the order of a minute to bash through a single reinforced wall, which is plenty of time to move to another location where it needs to bash through a new wall.
The "per inch" metric you're using comes from Wall of Stone, so it's not necessarily applicable to mundane walls. Per the DMG suggestions, a Large resilient object will have 5d10 HP, with 17 AC if it's made of stone. The exact substance of siege defenses is of course a case-by-case issue, but with a Tarrasque's damage output against objects, it's clearly meant to kaiju its way through anything short of plot-device level magic.
The "per inch" metric you're using comes from Wall of Stone, so it's not necessarily applicable to mundane walls.
The exact same text is in mighty fortress, and nothing about either spell in any way suggests that it's unusual stone.
Besides, you know, the fact that the stone can flow back into the earth if the spell is ended early or it's not made permanent, depending on the case. Spell descriptions describe the effects of spells, and they're not an authoritative source for general object interactions/properties. Honestly, they probably should have just taken a few paragraphs to describe some typical fortifications, though.
Let's examine the math for how this hypothetical flying bowman is driving off the Tarrasque.
First level options for doing it are very limited (an aarakokra cleric using sacred flame DC 13 can manage 1.62 dpr, so about 42 minutes), but by level 5 it's not really all that hard -- a level 5 hunter range with a +1 longbow (either because he has one, or because magic weapon) has attack +8 (20% vs AC 25, 5% to crit) for 1d8+1d6(hunter's mark)+1d8(colossus slayer)+5 = 17.5 (+12.5 on crit), 2 attacks (which very slightly reduces colossus slayer), overall dpr about 8.2, so a bit over 8 minutes -- less than the duration of fly. The aarakokra cleric (DC 15) now manages 4.41 dpr or a bit over 15 minutes; he could stack on spiritual weapon but with a duration of only a minute it's not terribly effective.
Is it a spectacularly boring and stupid fight? Yes, but that doesn't mean it won't work. Any threat that can be reliably dealt with by a party with an average level of less than half it's CR is badly designed (it's not the only badly designed monster in the MM, but it's an extreme outlier).
Let's examine the math for how this hypothetical flying bowman is driving off the Tarrasque.
First level options for doing it are very limited (an aarakokra cleric using sacred flame DC 13 can manage 1.62 dpr, so about 42 minutes), but by level 5 it's not really all that hard -- a level 5 hunter range with a +1 longbow (either because he has one, or because magic weapon) has attack +8 (20% vs AC 25, 5% to crit) for 1d8+1d6(hunter's mark)+1d8(colossus slayer)+5 = 17.5 (+12.5 on crit), 2 attacks (which very slightly reduces colossus slayer), overall dpr about 8.2, so a bit over 8 minutes -- less than the duration of fly. The aarakokra cleric (DC 15) now manages 4.41 dpr or a bit over 15 minutes; he could stack on [Tooltip Not Found] but with a duration of only a minute it's not terribly effective.
Is it a spectacularly boring and stupid fight? Yes, but that doesn't mean it won't work. Any threat that can be reliably dealt with by a party with an average level of less than half it's CR is badly designed (it's not the only badly designed monster in the MM, but it's an extreme outlier).
That's 8 minutes during which the Tarrasque is going to be doing an average of about 37000 HP of damage to structures in the area. That's over 260 Meteor Swarms worth of damage. I'd say one can safely extrapolate that into the utter devastation of anything short of the largest cities in a typical D&D setting. Honestly if you want to fix this issue, just slap a 10-20 HP regen on it, but regardless you're looking at a pyrrhic victory at best if you try to play the "kill it with bug bites" game in anything besides a white room scenario.
That's 8 minutes during which the Tarrasque is going to be doing an average of about 37000 HP of damage to structures in the area. That's over 260 Meteor Swarms worth of damage.
If meteor swarm was a single target spell, sure... but it isn't. It's also not that great a spell for laying waste to cities in the first place.
The Tarrasque isn't harmless, but it's not CR 30 worth of dangerous. It's generally easier to deal with than a CR 12 archdruid.
That's 8 minutes during which the Tarrasque is going to be doing an average of about 37000 HP of damage to structures in the area. That's over 260 Meteor Swarms worth of damage.
If meteor swarm was a single target spell, sure... but it isn't. It's also not that great a spell for laying waste to cities in the first place.
The Tarrasque isn't harmless, but it's not CR 30 worth of dangerous. It's generally easier to deal with than a CR 12 archdruid.
There is no good spell for laying waste to cities, which is rather my point. Yes, in a big empty chamber against a party that has specially tailored themselves to counter it it's going to be an underwhelming fight, but against a typical party in a scenario where they can't afford to take what is in combat terms a ridiculously long time to poke it to death, it's a pretty good juggernaut.
Honestly, Tarrasque is still coming out ahead imo. Any one attack from it hits about as hard as Earthquake or Tsunami, and it's got a lot more staying power. Tsunami in particular is a one and done for any single structure. They'd still leave a mark, but not nearly on the same scale a Tarrasque can. Your archdruid gets one minute of destruction once or twice a day. Even with the hypothetical flying magic bowman, a Tarrasque gets several times as long to play.
Honestly, Tarrasque is still coming out ahead imo. Any one attack from it hits about as hard as Earthquake or Tsunami, and it's got a lot more staying power.
Tsunami lays waste to 90,000 square feet. Assuming 1/3 of the town is building, that's 300 10x10 squares of building to destroy. The archdruid can cast it twice (using 8th and 9th level slots) from miles away (it has sight range) and then disappear. The Tarrasque typically takes around ten minutes to do that much damage.
If you want to instead compare monsters, a CR 27 red greatwyrm can destroy a 300' diameter area in one turn.
Honestly, Tarrasque is still coming out ahead imo. Any one attack from it hits about as hard as Earthquake or Tsunami, and it's got a lot more staying power.
Tsunami lays waste to 90,000 square feet. Assuming 1/3 of the town is building, that's 300 10x10 squares of building to destroy. The archdruid can cast it twice (using 8th and 9th level slots) from miles away (it has sight range) and then disappear. The Tarrasque typically takes around ten minutes to do that much damage.
If you want to instead compare monsters, a CR 27 red greatwyrm can destroy a 300' diameter area in one turn.
Tusnami scales down by a d10 every turn. It's got maybe three turns of laying waste, tops. And, no, Greatwyrms don't get a 300' diameter. It's a cone, that's a very different shape. Still very hefty damage, yeah, but they're only slightly below a Tarrasque on the CR scale as well and don't make like half the attack spells in the game just bounce away on top of magic resistance, so it ain't like the Tarrasque is short on tricks either.
Tusnami scales down by a d10 every turn. It's got maybe three turns of laying waste, tops. And, no, Greatwyrms don't get a 300' diameter. It's a cone, that's a very different shape. Still very hefty damage, yeah, but they're only slightly below a Tarrasque on the CR scale as well and don't make like half the attack spells in the game just bounce away on top of magic resistance, so it ain't like the Tarrasque is short on tricks either.
From 300' in the air, a 300' cone is a 300' diameter area. Greatwyrms are better at resistance to magic because, well, the saves on a Tarrasque are +10, +0, +10, +5, +9, +9 (total +43), the saves on a greatwyrm are +10, +10, +18, +5, +13, +16 (total +72), and advantage is only worth about +4, and of the important saves (Dex, Con, Wis) the greatwyrm is significantly ahead on two. The Tarrasque's reflective hide is nowhere near half the attack spells in the game, and most of the attack spells it does cover you wouldn't want to use anyway.
Huh, felt like there were a lot more ranged attack spells. You've got a point on the cone too, but I'm not sure how many people picture a cone like that. Regarding the saves, that average of +4 bumps them up to a total of +67, so by the numbers a Tarrasque is only slightly behind and advantage can also seriously swing any single roll, so that gives it some additional weight outside of the strictly numerical averages. And, at the end of the day, a Tarrasque is objectively better suited to smashing up terrain than a greatwyrm; they have much more consistent per round output and siege monster. Now, all this said, I think the Tarrasque definitely needs a Mythic Awakening ability to give it that extra kick, but unless you aggressively cherry pick a scenario to suit a particular narrative, the AC 25 immune to non-magic weapons kaiju with five to eight attacks a round is gonna tear up a city just as well as an uber dragon that's not even immune to normal arrows or a spell that only lasts a minute.
Don't listen to any of these people. You can kill the Tarrasque with 2 starting characters with starting gear and starting money if each of them has find familiar and a background that gives at least 10 gold to cast it. This runs a high chance of getting you kicked from your group because you are going to abuse 5e.
Step o. (cause I forgot some people are stupid and will get a familiar the tarrasque can attack and will 100% kill in 1 round.) Get flying familiars and make them stay at least 30 feet above the tarrasque when they get there.)
Step 1. Find the Tarrasque. Step 2. Tell the familiars to take turns making noise above the Terrasque so it can't sleep. One of them does it for 12 hours while the other sleeps, then they swap. Step 3. Go to a not so near by Inn and get drunk. Step 4. Stay drunk for about a week in which time the Tarrasque dies of exhaustion with no rolls, no save, no escape, no chance to avoid it because of how exhaustion works. Step 5. Buy a lot of beer and pizza for the party so they don't kick you out.
(Seriously, while this does work RAW, don't do it. It's on par with coffeelocks and artificier bag of holding nukes for how cheezy it is, and I can't claim credit for those two. This whole post is a joke.)
Even if they've got bagpipes, I don't think a couple of critters that could pass through the gaps in the Tarrasque's teeth could make enough noise to prevent it from sleeping.
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Find your own truth, choose your enemies carefully, and never deal with a dragon.
"Canon" is what's factual to D&D lore. "Cannon" is what you're going to be shot with if you keep getting the word wrong.
Technically you just need to keep ruining its rest with combat to make it keep starting over, and yeah, 6th day = 6th point of exhaustion (because they didn't even make it immune to exhaustion apparently).
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Playing D&D since 1982
Have played every version of the game since Basic (Red Box Set), except that abomination sometimes called 4e.
Let's examine the math for how this hypothetical flying bowman is driving off the Tarrasque.
For the same of argument, I'm going to assume that it's possible despite the Tarrasque being immune to fear, but it has to be brought down to half HP, which requires 336 points of damage.
Say the hypothetical archer has a +1 longbow with +7 to hit for 1d8+4 damage, average of 9. In order to hit the Tarrasque's 25 AC, they need to roll an 18 or better, so a 15% chance of hitting, and I'm going to ignore the effects of crits by assuming that they average out. That requires 37.4 successful hits- call it 37. And with a 15% accuracy rating, that works out to about 249 attacks. Just how many arrows is your archer carrying? Because that's 25 minutes worth of shooting at the thing assuming you never have to go pick up more ammo.
On top of everything else, firing a bow repeatedly is fairly strenuous activity. As a GM, I'd be inclined to start calling for constitution saving throws to avoid exhaustion after a few minutes. Less if the character were flying under their own power instead of by using magic.
Overall, the hypothetical situation of the lone/small group of flying archers stopping the Tarrasque by themselves is honestly not realistic.
Find your own truth, choose your enemies carefully, and never deal with a dragon.
"Canon" is what's factual to D&D lore. "Cannon" is what you're going to be shot with if you keep getting the word wrong.
Experimenting with how I might design a tarrasque that was actually a proper terror
Charger. If the Tarrasque moves at least 20' towards a target and then hits it with a Horns attack on the same turn, the target takes an additional 22 (4d10) piercing damage and must make a Strength save (DC 27) or be knocked prone.
Damage Threshold. If damage to the Tarrasque from a single character or other effect is less than 30 in a single action, the Tarrasque takes no damage.
Elder of Annihilation. The Tarrasque's damage cannot be reduced by resistance or immunity. If the Tarrasque attacks a force object, that object is destroyed.
Juggernaut. No form of terrain costs the Tarrasque extra movement to pass through. No effect can reduce the Tarrasque's speed. The Tarrasque may use its burrow speed to move through solid objects as long as they are no tougher than stone.
Legendary Recovery. If the Tarrasque takes no action on its turn, or has its action compelled by an external effect, it reduces the duration of any number of effects on it by one round and may make a new save against any remaining effects, even if that effect does not normally allow repeated saves. It then heals by 80 hp. The Tarrasque may also trigger this at the end of another creature's turn by spending 2 legendary actions.
Legendary Regeneration. If the Tarrasque starts its turn with unspent legendary actions, it heals by 40 hp per unspent action.
Reflective Carapace. Any time the Tarrasque is targeted by a magic missile spell, a line spell, or a spell that requires a ranged attack roll, roll a d6. On a 1 to 5, the tarrasque is unaffected. On a 6, the Tarrasque is unaffected, and the effect is reflected back at the caster as though it originated from the Tarrasque, turning the caster into the target.
Save Substitution. The Tarrasque may use its its proficiency bonus in place of any save.
Siege Monster. The Tarrasque deals double damage to objects and structures.
Swath of Destruction. If the Tarrasque moves within 5' of a creature or object, that object takes 10 (3d6) damage, but no more than once per turn. Creatures may make a Dexterity save (DC 27) to avoid this damage.
Unkillable. The Tarrasque cannot die.
Arena of Earth (Recharges after ten minutes). If the Tarrasque would be reduced to 0 hit points, its current hit point total instead resets to 600, and for the next ten minutes the earth shakes, as per Earthquake (save DC 22), though it does not count as a spell and does not require concentration. This area is surrounded by a dome of stone 5' thick (treat as a wall of stone: AC 15, 30 hp per inch). Two fissures (as the lair action below) appear within the area. For the duration of the quake, the Tarrasque is considered to be within its lair and may use lair actions. If the Tarrasque is reduced to zero hit points during the quake, a fissure appears under it, it falls in, and is entombed, as if affected by the burial effect of the imprisonment spell. Award a party an additional 155,000 XP (310,000 XP total) for defeating the Tarrasque within the arena of earth.
Multiattack. The Tarrasque can use its Frightful Presence, uses one of Bite, Swallow, and Horns, and makes two Claw attacks.
Bite. Melee Weapon Attack: +19 to hit, reach 10 ft., one target. Hit: 36 (4d12 + 10) piercing damage. If the target is a creature, it is grappled (escape DC 27). Until this grapple ends, the target is restrained, and the Tarrasque can’t bite another target.
Claw. Melee Weapon Attack: +19 to hit, reach 15ft., one target. Hit: 28 (4d8 + 10) slashing damage.
Horns. Melee Weapon Attack: +19 to hit, reach 10ft., one target. Hit: 32 (4d10 + 10) piercing damage.
Roar (Recharge 5-6). All other creatures with 120' of the Tarrasque must make a Constitution save (DC 27) or suffer 22 (4d10) thunder damage and be stunned until the end of the Tarrasque's next turn. Creatures that succeed at the save take half damage and are not stunned.
Tail. Melee Weapon Attack: +19 to hit, reach 20ft., one target. Hit: 24 (4d6 + 10) bludgeoning damage. If the target is a creature, it must succeed on a DC 27 Strength saving throw or be shoved back up to 20' and knocked prone.
Frightful Presence. Each creature of the Tarrasque’s choice within 120 feet of it and aware of it must succeed on a DC 22 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, with disadvantage if the Tarrasque is within line of sight, ending the effect on itself on a success. If a creature’s saving throw is successful or the effect ends for it, the creature is immune to the Tarrasque’s Frightful Presence for the next 24 hours.
Swallow. One Large or smaller creature the Tarrasque is grappling must make a Strength save (DC 27) or be swallowed, ending the grapple. While swallowed, the creature is blinded and restrained, it has total cover against attacks and other effects outside the Tarrasque, and it takes 56 (16d6) acid damage at the start of each of the Tarrasque’s turns.
If the Tarrasque takes 60 damage or more on a single turn from a creature inside it, the Tarrasque must succeed on a DC 20 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of the Tarrasque. If the Tarrasque is reduced to 0 hp, all swallowed creatures are automatically regurgitated.
The Tarrasque can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. The tarrasque regains spent legendary actions at the start of its turn.
Charge (costs 2 Actions). The Tarrasque moves up to its speed and uses Horns.
Chomp (Costs 2 Actions). The Tarrasque uses Bite or Swallow.
Earthbind. One creature within 300' that the Tarrasque can see must make a Constitution save (DC 27) or lose the ability to fly for one minute, possibly causing it to fall. An affected creature may repeat the save at the end of each of its turn, ending the effect on a success.
Move. The Tarrasque moves up to half its speed, and gains 40 temporary hit points.
Tail Swipe. The Tarrasque makes a tail attack.
Lair and Lair Actions
On initiative count 20 (losing initiative ties), the Tarrasque takes one of the following actions
Open Fissure: a fissure 10' wide, 60' deep, and extending the length of the arena appears. Creatures in the area of the fissure when it opens must make a Dexterity save (DC 27) or fall in, taking 21 (6d6) falling damage. Structures in the area of the fissure are destroyed.
Close Fissure: One fissure within the arena closes. Any creature in the fissure at the time it closes suffers 21 (6d6) bludgeoning damage and must make a Dexterity save (DC 27) or be restrained; whether or not it is restrained, it lacks line of sight on any other creature until it exits the fissure. A restrained creature may spend its action to make an acrobatics or athletics check (DC 20); a creature that is not restrained may climb towards the surface but treats the fissure as difficult terrain in addition to any climbing penalties.
Eruption: a geyser of lava erupts from the ground, in a cylinder 10' wide and extending 20' above the ground; if the eruption occurs over a fissure, it also extends to the bottom. Any creature in the area must make a Dexterity save (DC 27) or suffer 21 (6d6) fire damage; on a successful save, damage is halved. The geyser lasts until the next lair action; any creature that ends its the area must make the same save.
Haven't really crunched the CR numbers so it might be off. Its raw damage output is I think lower than vanilla.
The "per inch" metric you're using comes from Wall of Stone, so it's not necessarily applicable to mundane walls. Per the DMG suggestions, a Large resilient object will have 5d10 HP, with 17 AC if it's made of stone. The exact substance of siege defenses is of course a case-by-case issue, but with a Tarrasque's damage output against objects, it's clearly meant to kaiju its way through anything short of plot-device level magic.
The exact same text is in mighty fortress, and nothing about either spell in any way suggests that it's unusual stone.
Besides, you know, the fact that the stone can flow back into the earth if the spell is ended early or it's not made permanent, depending on the case. Spell descriptions describe the effects of spells, and they're not an authoritative source for general object interactions/properties. Honestly, they probably should have just taken a few paragraphs to describe some typical fortifications, though.
First level options for doing it are very limited (an aarakokra cleric using sacred flame DC 13 can manage 1.62 dpr, so about 42 minutes), but by level 5 it's not really all that hard -- a level 5 hunter range with a +1 longbow (either because he has one, or because magic weapon) has attack +8 (20% vs AC 25, 5% to crit) for 1d8+1d6(hunter's mark)+1d8(colossus slayer)+5 = 17.5 (+12.5 on crit), 2 attacks (which very slightly reduces colossus slayer), overall dpr about 8.2, so a bit over 8 minutes -- less than the duration of fly. The aarakokra cleric (DC 15) now manages 4.41 dpr or a bit over 15 minutes; he could stack on spiritual weapon but with a duration of only a minute it's not terribly effective.
Is it a spectacularly boring and stupid fight? Yes, but that doesn't mean it won't work. Any threat that can be reliably dealt with by a party with an average level of less than half it's CR is badly designed (it's not the only badly designed monster in the MM, but it's an extreme outlier).
That's 8 minutes during which the Tarrasque is going to be doing an average of about 37000 HP of damage to structures in the area. That's over 260 Meteor Swarms worth of damage. I'd say one can safely extrapolate that into the utter devastation of anything short of the largest cities in a typical D&D setting. Honestly if you want to fix this issue, just slap a 10-20 HP regen on it, but regardless you're looking at a pyrrhic victory at best if you try to play the "kill it with bug bites" game in anything besides a white room scenario.
If meteor swarm was a single target spell, sure... but it isn't. It's also not that great a spell for laying waste to cities in the first place.
The Tarrasque isn't harmless, but it's not CR 30 worth of dangerous. It's generally easier to deal with than a CR 12 archdruid.
There is no good spell for laying waste to cities, which is rather my point. Yes, in a big empty chamber against a party that has specially tailored themselves to counter it it's going to be an underwhelming fight, but against a typical party in a scenario where they can't afford to take what is in combat terms a ridiculously long time to poke it to death, it's a pretty good juggernaut.
Um.. there's a reason I mentioned archdruid. Earthquake is designed for the purpose, tsunami doesn't specify working on objects but reasonably should.
Honestly, Tarrasque is still coming out ahead imo. Any one attack from it hits about as hard as Earthquake or Tsunami, and it's got a lot more staying power. Tsunami in particular is a one and done for any single structure. They'd still leave a mark, but not nearly on the same scale a Tarrasque can. Your archdruid gets one minute of destruction once or twice a day. Even with the hypothetical flying magic bowman, a Tarrasque gets several times as long to play.
Tsunami lays waste to 90,000 square feet. Assuming 1/3 of the town is building, that's 300 10x10 squares of building to destroy. The archdruid can cast it twice (using 8th and 9th level slots) from miles away (it has sight range) and then disappear. The Tarrasque typically takes around ten minutes to do that much damage.
If you want to instead compare monsters, a CR 27 red greatwyrm can destroy a 300' diameter area in one turn.
Tusnami scales down by a d10 every turn. It's got maybe three turns of laying waste, tops. And, no, Greatwyrms don't get a 300' diameter. It's a cone, that's a very different shape. Still very hefty damage, yeah, but they're only slightly below a Tarrasque on the CR scale as well and don't make like half the attack spells in the game just bounce away on top of magic resistance, so it ain't like the Tarrasque is short on tricks either.
From 300' in the air, a 300' cone is a 300' diameter area. Greatwyrms are better at resistance to magic because, well, the saves on a Tarrasque are +10, +0, +10, +5, +9, +9 (total +43), the saves on a greatwyrm are +10, +10, +18, +5, +13, +16 (total +72), and advantage is only worth about +4, and of the important saves (Dex, Con, Wis) the greatwyrm is significantly ahead on two. The Tarrasque's reflective hide is nowhere near half the attack spells in the game, and most of the attack spells it does cover you wouldn't want to use anyway.
Huh, felt like there were a lot more ranged attack spells. You've got a point on the cone too, but I'm not sure how many people picture a cone like that. Regarding the saves, that average of +4 bumps them up to a total of +67, so by the numbers a Tarrasque is only slightly behind and advantage can also seriously swing any single roll, so that gives it some additional weight outside of the strictly numerical averages. And, at the end of the day, a Tarrasque is objectively better suited to smashing up terrain than a greatwyrm; they have much more consistent per round output and siege monster. Now, all this said, I think the Tarrasque definitely needs a Mythic Awakening ability to give it that extra kick, but unless you aggressively cherry pick a scenario to suit a particular narrative, the AC 25 immune to non-magic weapons kaiju with five to eight attacks a round is gonna tear up a city just as well as an uber dragon that's not even immune to normal arrows or a spell that only lasts a minute.
Don't listen to any of these people. You can kill the Tarrasque with 2 starting characters with starting gear and starting money if each of them has find familiar and a background that gives at least 10 gold to cast it. This runs a high chance of getting you kicked from your group because you are going to abuse 5e.
Step o. (cause I forgot some people are stupid and will get a familiar the tarrasque can attack and will 100% kill in 1 round.) Get flying familiars and make them stay at least 30 feet above the tarrasque when they get there.)
Step 1. Find the Tarrasque.
Step 2. Tell the familiars to take turns making noise above the Terrasque so it can't sleep. One of them does it for 12 hours while the other sleeps, then they swap.
Step 3. Go to a not so near by Inn and get drunk.
Step 4. Stay drunk for about a week in which time the Tarrasque dies of exhaustion with no rolls, no save, no escape, no chance to avoid it because of how exhaustion works.
Step 5. Buy a lot of beer and pizza for the party so they don't kick you out.
(Seriously, while this does work RAW, don't do it. It's on par with coffeelocks and artificier bag of holding nukes for how cheezy it is, and I can't claim credit for those two. This whole post is a joke.)
Even if they've got bagpipes, I don't think a couple of critters that could pass through the gaps in the Tarrasque's teeth could make enough noise to prevent it from sleeping.
Find your own truth, choose your enemies carefully, and never deal with a dragon.
"Canon" is what's factual to D&D lore. "Cannon" is what you're going to be shot with if you keep getting the word wrong.
Technically you just need to keep ruining its rest with combat to make it keep starting over, and yeah, 6th day = 6th point of exhaustion (because they didn't even make it immune to exhaustion apparently).
Playing D&D since 1982
Have played every version of the game since Basic (Red Box Set), except that abomination sometimes called 4e.
Yeah, good luck kiting it for six days without getting eaten.
Find your own truth, choose your enemies carefully, and never deal with a dragon.
"Canon" is what's factual to D&D lore. "Cannon" is what you're going to be shot with if you keep getting the word wrong.
That's why they have to be flying familiars. The Terrasque doesn't have a ranged attack. And this should get you kicked out of any table.