I know I made a post like this before, but I didn't really get a good answer last time, so we're doing it again!
The question is, how do you calculate the challenge rating of a character who was created using player character creation, specifically a Fathomless Warlock? I'm wanting for him to level up as the party does, getting stronger at the same time as them. Sort of like a scaling enemy.
Can you share the specific character? a lot of the CR rating of spellcasters is factoring in the damage potential of their spells. Without knowing what spells the Warlock has, it will be very difficult to calculate a CR.
As a (very general) rule, the CR for PCs should be about 3-4 stages below PC level (at lower levels) and about 1/2 the PC Level (for higher levels), but again, that depends highly on spells for spellcasting classes.
I'm more than happy to help if you can share the character sheet.
Honestly, the few answers you got before were on point. Any general rule of thumb like CR = 1/4 level for the first 5 levels, level -2 for the next 5, level -4 for the last ten is going to be awfully rough. It depends - a lot - on how the NPC is built and what gear they have. CRs are approximations anyway, and if you’d want them more or less reliably close to what they’d officially be, you’d need to do the actual calculations based on the actual stat block. There’s no dependable shortcut.
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When I create a villain using PC rules, I usually put them about 2 levels ahead of the party, but (assuming they're the main threat in the encounter), I multiply their hit points by the number of characters in the party. PCs as written are glass cannons and, while they might be okay foes as a group, make terrible bosses. I also add any healing (Lay on Hands, Second Wind) to their hit points before multiplying, in order to help with their action economy. And if the party has a dedicated controller, it's good to give them a way to resist magic (like a Legendary Resistance) so that they don't get stunlocked.
Anyway, I know that's a roundabout answer, but it's really the most help I can give.
Trying to build an NPC to match the CR capabilities of the players is a fool's errand. Trust me, I have tried. Initiative alone can change an encounter from easy to deadly, at least with spell caster NPC's. Using a PC build as a base line is fine, but you need to add minions, lair actions, and Legendary Actions, even for a low level encounter.
Okay, here is the long method for working out how to calculate the CR. I've done this multiple times for a friends Warlock and it's always a pain in the arse, not as bad as Druid's though.
1. Work out your warlocks 'effective hitpoints'. Effective hp is your characters hp after it has been modified based on any active effects or spells it may cast before combat starts. The only racial trait (Off the top of my head) that affects this are the Lizardfolks bite attack and the Half-Orc's relentless ability, but check your racial abilities just in case. If the warlock plans on casting False Life before combat starts you'd need to factor that in to get the effective 'hitpoints'. Certain subclasses grant abilities that can increase hp as well, like the Celestial's Healing Light ability. The other thing that can affect this is resistances, for example given by spells like Stoneskin, you only need to change the hp if it has at least 3 resistances or immunities, consult the resistance table in the create a monster section of the DMG to determine that.
2. Work out your warlock's 'effective AC'. Remember to consider whether your warlock casts Mage Armor or a similar ac increasing spell beforehand. It's unlikely, but if your warlock plans to cast shield for 3 rounds then the effective ac would increase by 5, as a warlock has very few spell slots, it's safe to assume this won't be the strategy. Being able to fly increases the effective ac by +2, any effect that would cause enemies to have disadvantage when attacking the warlock (Magic Resistance, Evasion, Greater Invisibility etc) also increase the effective ac by +2.
3. Go to the top of the Create a Monster section in the DMG to calculate the defensive CR. This is calculated by going down the table until you reach the row with your warlocks effective hp. Look at the AC column to the left of that, for every 2 points of ac their are different between the AC given in that row and the effective ac of the warlock, move up or down 1 row. This will give you your defensive CR.
4. Now it's the complicated bit, calculating damage. This is calculated by working out the highest amount of damage the warlock can do in 3 rounds of combat when using the average damage values for those attacks/spells then dividing the result by 3. You ALWAYS assume the attack/spell hits, the target(s) fails their saving throw and that any spell with an area of effect (Line, Radius, Cube etc) hits 2 targets. For a warlock, work out how many spellslots they have left if they used up any casting False Life or Mage Armor before combat starts, then work out what the most damaging spells are. At level 5 for example Fireball and Lightning Bolt are the highest damage dealing spells. Once you've burned up your spell slots, Eldritch blast is (usually) the best option damage dealing wise, remember warlocks fire more eldritch blasts at higher levels. Factor in any abilities that might effect the damage of your spells, such as the Agonizing Blast eldritch invocation. It is assumed the warlock gets attacked once per round, so if they have reaction spells like Hellish Rebuke you can factor in those when calculating the damage if necessary.
5. Work out the effective attack bonus. Unless the warlock has an ability that allows them to frequently have advantage on their attacks (Pack Tactics etc), the effective attack bonus will just be the charisma modifier plus proficiency bonus. If it has an ability like pack tactics, you add +1, if it ALWAYS has advantage on their attacks then you add +4. (I don't know why the difference is so huge, this is just what it gives in the DMG)
6. Now we work out the Offensive CR. Go back up to the table and go down the damage row until you reach the calculated average damager per round. Then look to the left of that, for every 2 points of difference between the attack bonus on that row and the effective attack bonus you calculated, go up or down the table by 1 row. Look to the right to find the offensive CR.
7. Add the defensive CR and offensive CR together then divide by 2 and round down. That's your warlocks CR.
As you can see, this method is insanely long and also includes quite a few assumptions as the DMG doesn't explain a lot of things. From experience, a warlocks CR is usually 1/3 of their level if they have very few offensive spells or 2/3 of their level if they have quite a few offensive options. I would advise ignoring the headache inducing method shown above and if you have access to it, use the Sidekick option provided in Tasha's.
Okay, here is the long method for working out how to calculate the CR. I've done this multiple times for a friends Warlock and it's always a pain in the arse, not as bad as Druid's though.
1. Work out your warlocks 'effective hitpoints'. Effective hp is your characters hp after it has been modified based on any active effects or spells it may cast before combat starts. The only racial trait (Off the top of my head) that affects this are the Lizardfolks bite attack and the Half-Orc's relentless ability, but check your racial abilities just in case. If the warlock plans on casting False Life before combat starts you'd need to factor that in to get the effective 'hitpoints'. Certain subclasses grant abilities that can increase hp as well, like the Celestial's Healing Light ability. The other thing that can affect this is resistances, for example given by spells like Stoneskin, you only need to change the hp if it has at least 3 resistances or immunities, consult the resistance table in the create a monster section of the DMG to determine that.
2. Work out your warlock's 'effective AC'. Remember to consider whether your warlock casts Mage Armor or a similar ac increasing spell beforehand. It's unlikely, but if your warlock plans to cast shield for 3 rounds then the effective ac would increase by 5, as a warlock has very few spell slots, it's safe to assume this won't be the strategy. Being able to fly increases the effective ac by +2, any effect that would cause enemies to have disadvantage when attacking the warlock (Magic Resistance, Evasion, Greater Invisibility etc) also increase the effective ac by +2.
3. Go to the top of the Create a Monster section in the DMG to calculate the defensive CR. This is calculated by going down the table until you reach the row with your warlocks effective hp. Look at the AC column to the left of that, for every 2 points of ac their are different between the AC given in that row and the effective ac of the warlock, move up or down 1 row. This will give you your defensive CR.
4. Now it's the complicated bit, calculating damage. This is calculated by working out the highest amount of damage the warlock can do in 3 rounds of combat when using the average damage values for those attacks/spells then dividing the result by 3. You ALWAYS assume the attack/spell hits, the target(s) fails their saving throw and that any spell with an area of effect (Line, Radius, Cube etc) hits 2 targets. For a warlock, work out how many spellslots they have left if they used up any casting False Life or Mage Armor before combat starts, then work out what the most damaging spells are. At level 5 for example Fireball and Lightning Bolt are the highest damage dealing spells. Once you've burned up your spell slots, Eldritch blast is (usually) the best option damage dealing wise, remember warlocks fire more eldritch blasts at higher levels. Factor in any abilities that might effect the damage of your spells, such as the Agonizing Blast eldritch invocation. It is assumed the warlock gets attacked once per round, so if they have reaction spells like Hellish Rebuke you can factor in those when calculating the damage if necessary.
5. Work out the effective attack bonus. Unless the warlock has an ability that allows them to frequently have advantage on their attacks (Pack Tactics etc), the effective attack bonus will just be the charisma modifier plus proficiency bonus. If it has an ability like pack tactics, you add +1, if it ALWAYS has advantage on their attacks then you add +4. (I don't know why the difference is so huge, this is just what it gives in the DMG)
6. Now we work out the Offensive CR. Go back up to the table and go down the damage row until you reach the calculated average damager per round. Then look to the left of that, for every 2 points of difference between the attack bonus on that row and the effective attack bonus you calculated, go up or down the table by 1 row. Look to the right to find the offensive CR.
7. Add the defensive CR and offensive CR together then divide by 2 and round down. That's your warlocks CR.
As you can see, this method is insanely long and also includes quite a few assumptions as the DMG doesn't explain a lot of things. From experience, a warlocks CR is usually 1/3 of their level if they have very few offensive spells or 2/3 of their level if they have quite a few offensive options. I would advise ignoring the headache inducing method shown above and if you have access to it, use the Sidekick option provided in Tasha's.
I tried this method, and he has a challenge rating of 10 lol
So, Baseline Warlock with those spells would have 8 + 9d8 + 10xCon (78 average with +3 to CON for this example), +9 to hit with spells and a DC 17 spell save (assuming maxed CHA by level 10), 2 5th level spell slots and a damaging bonus action attack with the tentacle. Assuming +3 Dex and Studded Leather, that gives AC of 15 and Cold Resistance from subclass. Racial abilities and Saving Throw Proficiencies don't exceed the limits of what would affect CR so lets start
Defensive CR: Hitpoints place you squarely in CR 1 territory, and even if you do the multiplier for cold resistance (which is not recommended as you usually do this when you have multiple resistances and/or more common ones) that is CR 4. Because its only 1 relatively uncommon resistance, we will start at CR 1. By casting Everards Tentacles you gain 10 temp HP which would raise that to CR 2. expected AC for that CR would be 13; because yours is 15 that increases Defensive CR by 1, so the final Defensive CR is CR 3
Offensive CR: The damage table assumes 1) you use your most damaging available attacks on your turn for average damage, 2) combat lasts for 3 rounds, and 3) attack rolls hit and AoE effects affect 2 creatures who fail their saves.. With these, your most damaging Turn choices would be
Turn 1: Bonus Action Tentacle (2d8 = 9) + Everards Tentacles (3d6 x 2 = 20) for 29 damage (no spell slot use, subclass ability)
Turn 2: Bonus Action Tentacle (2d8 = 9) + Cone of Cold (8d8 x 2 = 72) + Everards additional damage that round (3d6 x 2 = 20) for 101 damage (spell slot use)
Turn 3: Bonus Action Tentacle (2d8 = 9) + Cone of Cold (8d8 x2 = 72) + Everards additional damage that round (3d6 x 2 = 20) for 101 damage (spell slot use)
Average damage for that is 77 or CR 12. Note that Lightning Bolt is (10d6 x 2 = 70) and somewhat comparable to Cone of Cold if used in place of it, and no other spell options you have listed deal close to that amount of damage per round. The expected Save DC and Attack Bonus is 17 and +8 respectively, your 17 / + 9 does not change that number, so Offensive CR is CR 12
Bonuses: If any Feats or Eldritch Invocations would add additional saving throw proficiencies, resistances, or mimic an ability from the DMG Chapter 9 "Monster Features" table found here https://www.dndbeyond.com/sources/dmg/dungeon-masters-workshop#CreatingaMonsterStatBlock then you need to adjust the values above as stated. But the next step is averaging out the Offensive and Defensive CRs (12 + 3) / 2 = 7.5 rounded to CR 7
That Said, this statblock is a bit of a glass cannon (High Offense, Low Defense), so it may not last 3 rounds (or even 1) without some form of boost to AC, Saving Throws, Resistances, or additional HP. That will change defensive CR and total CR, so be aware of that.
I ended up with the same defensive CR as you, but a different offensive CR. Using Evard's Black Tentacles as my first action, also using Tentacle of the Deeps as my bonus action each turn, but used Lightning Bolt as the damage spell for that turn. 60 damage for each of the two targets, times 2 for the two castings, is 240 for the lightning bolts alone. Tentacle of the Deeps does a total of 48 damage, plus the damage from Evard's Black Tentacles (which is 18 for each of the two targets for each of the three rounds, adds up to 108) adding up to a total of 396. 396 divided by 3 is 132. With 20 Charisma and a PB of +4, the attack modifier is +9, which would take the Offensive CR to about 18. 18 + 3 = 21, 21/2 ~= 10.
I ended up with the same defensive CR as you, but a different offensive CR. Using Evard's Black Tentacles as my first action, also using Tentacle of the Deeps as my bonus action each turn, but used Lightning Bolt as the damage spell for that turn. 60 damage for each of the two targets, times 2 for the two castings, is 240 for the lightning bolts alone. Tentacle of the Deeps does a total of 48 damage, plus the damage from Evard's Black Tentacles (which is 18 for each of the two targets for each of the three rounds, adds up to 108) adding up to a total of 396. 396 divided by 3 is 132. With 20 Charisma and a PB of +4, the attack modifier is +9, which would take the Offensive CR to about 18. 18 + 3 = 21, 21/2 ~= 10.
Your mistake was using max damage for your spells and attacks instead of Average Damage. the calculation method says to use average damage. The average damage of 10d6 is 35, times 2 for 70 per round used. The average damage of Everards 3d6 is 10, times 2 for 20, and the average damage of the tentacles 2d8 is 9 damage.
The average damage for dice is as follows: 2.5 for a d4, 3.5 for a d6, 4.5 for a d8, 6.5 for a d12, and 10.5 for a d20. The .5 is because there is no 0 on the dice.
I ended up with the same defensive CR as you, but a different offensive CR. Using Evard's Black Tentacles as my first action, also using Tentacle of the Deeps as my bonus action each turn, but used Lightning Bolt as the damage spell for that turn. 60 damage for each of the two targets, times 2 for the two castings, is 240 for the lightning bolts alone. Tentacle of the Deeps does a total of 48 damage, plus the damage from Evard's Black Tentacles (which is 18 for each of the two targets for each of the three rounds, adds up to 108) adding up to a total of 396. 396 divided by 3 is 132. With 20 Charisma and a PB of +4, the attack modifier is +9, which would take the Offensive CR to about 18. 18 + 3 = 21, 21/2 ~= 10.
Your mistake was using max damage for your spells and attacks instead of Average Damage. the calculation method says to use average damage. The average damage of 10d6 is 35, times 2 for 70 per round used. The average damage of Everards 3d6 is 10, times 2 for 20, and the average damage of the tentacles 2d8 is 9 damage.
The average damage for dice is as follows: 2.5 for a d4, 3.5 for a d6, 4.5 for a d8, 6.5 for a d12, and 10.5 for a d20. The .5 is because there is no 0 on the dice.
I ended up with the same defensive CR as you, but a different offensive CR. Using Evard's Black Tentacles as my first action, also using Tentacle of the Deeps as my bonus action each turn, but used Lightning Bolt as the damage spell for that turn. 60 damage for each of the two targets, times 2 for the two castings, is 240 for the lightning bolts alone. Tentacle of the Deeps does a total of 48 damage, plus the damage from Evard's Black Tentacles (which is 18 for each of the two targets for each of the three rounds, adds up to 108) adding up to a total of 396. 396 divided by 3 is 132. With 20 Charisma and a PB of +4, the attack modifier is +9, which would take the Offensive CR to about 18. 18 + 3 = 21, 21/2 ~= 10.
Your mistake was using max damage for your spells and attacks instead of Average Damage. the calculation method says to use average damage. The average damage of 10d6 is 35, times 2 for 70 per round used. The average damage of Everards 3d6 is 10, times 2 for 20, and the average damage of the tentacles 2d8 is 9 damage.
The average damage for dice is as follows: 2.5 for a d4, 3.5 for a d6, 4.5 for a d8, 6.5 for a d12, and 10.5 for a d20. The .5 is because there is no 0 on the dice.
ohhh ok thx
Youre welcome! Its a complicated process, even more so for spellcasters since they have so many attack options, and its especially swingy for NPC's with levels since their hitpoints are always lower than a typical monster's hitpoints
That Said, this statblock is a bit of a glass cannon (High Offense, Low Defense), so it may not last 3 rounds (or even 1) without some form of boost to AC, Saving Throws, Resistances, or additional HP. That will change defensive CR and total CR, so be aware of that.
This is exactly why high CR monsters often have legendary saves, and why using PC rules to create NPCs isn't ideal.
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That Said, this statblock is a bit of a glass cannon (High Offense, Low Defense), so it may not last 3 rounds (or even 1) without some form of boost to AC, Saving Throws, Resistances, or additional HP. That will change defensive CR and total CR, so be aware of that.
This is exactly why high CR monsters often have legendary saves, and why using PC rules to create NPCs isn't ideal.
To be precise, how would a (1) Legendary Resistance affect the challenge rating of this creature? Assuming this is using a homebrew magic item to gain this legendary resistance.
That Said, this statblock is a bit of a glass cannon (High Offense, Low Defense), so it may not last 3 rounds (or even 1) without some form of boost to AC, Saving Throws, Resistances, or additional HP. That will change defensive CR and total CR, so be aware of that.
This is exactly why high CR monsters often have legendary saves, and why using PC rules to create NPCs isn't ideal.
To be precise, how would a (1) Legendary Resistance affect the challenge rating of this creature? Assuming this is using a homebrew magic item to gain this legendary resistance.
For your warlock, each use of legendary resistance it has per day increases it's effective hitpoints by 20hp per use
That Said, this statblock is a bit of a glass cannon (High Offense, Low Defense), so it may not last 3 rounds (or even 1) without some form of boost to AC, Saving Throws, Resistances, or additional HP. That will change defensive CR and total CR, so be aware of that.
This is exactly why high CR monsters often have legendary saves, and why using PC rules to create NPCs isn't ideal.
To be precise, how would a (1) Legendary Resistance affect the challenge rating of this creature? Assuming this is using a homebrew magic item to gain this legendary resistance.
For your warlock, each use of legendary resistance it has per day increases it's effective hitpoints by 20hp per use
With this addition, the overall CR would change to be 9. Thanks!
That Said, this statblock is a bit of a glass cannon (High Offense, Low Defense), so it may not last 3 rounds (or even 1) without some form of boost to AC, Saving Throws, Resistances, or additional HP. That will change defensive CR and total CR, so be aware of that.
This is exactly why high CR monsters often have legendary saves, and why using PC rules to create NPCs isn't ideal.
To be precise, how would a (1) Legendary Resistance affect the challenge rating of this creature? Assuming this is using a homebrew magic item to gain this legendary resistance.
For your warlock, each use of legendary resistance it has per day increases it's effective hitpoints by 20hp per use
With this addition, the overall CR would change to be 9. Thanks!
Legendary resistance alone doesn't exactly fix the glass cannon issue, I just brought it up as an obvious example. The larger point is that encounters with opponents that are heavy on offense and light on defense can be a bit volatile - if the PCs get lucky it'll be over really quickly and the encounter will be an anticlimax, if they're unlucky the whole thing can get out of hand fast. Nonetheless, you do you - it's your campaign, you know best.
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I know I made a post like this before, but I didn't really get a good answer last time, so we're doing it again!
The question is, how do you calculate the challenge rating of a character who was created using player character creation, specifically a Fathomless Warlock? I'm wanting for him to level up as the party does, getting stronger at the same time as them. Sort of like a scaling enemy.
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Spells, Monsters, Magic Items, Feats, Subclasses.
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Extended Signature
Can you share the specific character? a lot of the CR rating of spellcasters is factoring in the damage potential of their spells. Without knowing what spells the Warlock has, it will be very difficult to calculate a CR.
As a (very general) rule, the CR for PCs should be about 3-4 stages below PC level (at lower levels) and about 1/2 the PC Level (for higher levels), but again, that depends highly on spells for spellcasting classes.
I'm more than happy to help if you can share the character sheet.
Honestly, the few answers you got before were on point. Any general rule of thumb like CR = 1/4 level for the first 5 levels, level -2 for the next 5, level -4 for the last ten is going to be awfully rough. It depends - a lot - on how the NPC is built and what gear they have. CRs are approximations anyway, and if you’d want them more or less reliably close to what they’d officially be, you’d need to do the actual calculations based on the actual stat block. There’s no dependable shortcut.
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When I create a villain using PC rules, I usually put them about 2 levels ahead of the party, but (assuming they're the main threat in the encounter), I multiply their hit points by the number of characters in the party. PCs as written are glass cannons and, while they might be okay foes as a group, make terrible bosses. I also add any healing (Lay on Hands, Second Wind) to their hit points before multiplying, in order to help with their action economy. And if the party has a dedicated controller, it's good to give them a way to resist magic (like a Legendary Resistance) so that they don't get stunlocked.
Anyway, I know that's a roundabout answer, but it's really the most help I can give.
Wizard (Gandalf) of the Tolkien Club
Trying to build an NPC to match the CR capabilities of the players is a fool's errand. Trust me, I have tried. Initiative alone can change an encounter from easy to deadly, at least with spell caster NPC's. Using a PC build as a base line is fine, but you need to add minions, lair actions, and Legendary Actions, even for a low level encounter.
As a 10th-level Fathomless Warlock, he would have evard's black tentacles (Grasping Tentacles), create or destroy water, silence, lightning bolt, control water, summon elemental, cone of cold, far step, elemental bane, tongues, scrying, toll the dead, prestidigitation, minor illusion, mage hand (Known Spells), shape water, message, thaumaturgy (Tome), gust (High Elf)
P.S. I wouldn't be sending him ALONE against the party. The premise is that he's the leader of a cult.
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My homebrew stuff:
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Okay, here is the long method for working out how to calculate the CR. I've done this multiple times for a friends Warlock and it's always a pain in the arse, not as bad as Druid's though.
1. Work out your warlocks 'effective hitpoints'. Effective hp is your characters hp after it has been modified based on any active effects or spells it may cast before combat starts. The only racial trait (Off the top of my head) that affects this are the Lizardfolks bite attack and the Half-Orc's relentless ability, but check your racial abilities just in case. If the warlock plans on casting False Life before combat starts you'd need to factor that in to get the effective 'hitpoints'. Certain subclasses grant abilities that can increase hp as well, like the Celestial's Healing Light ability. The other thing that can affect this is resistances, for example given by spells like Stoneskin, you only need to change the hp if it has at least 3 resistances or immunities, consult the resistance table in the create a monster section of the DMG to determine that.
2. Work out your warlock's 'effective AC'. Remember to consider whether your warlock casts Mage Armor or a similar ac increasing spell beforehand. It's unlikely, but if your warlock plans to cast shield for 3 rounds then the effective ac would increase by 5, as a warlock has very few spell slots, it's safe to assume this won't be the strategy. Being able to fly increases the effective ac by +2, any effect that would cause enemies to have disadvantage when attacking the warlock (Magic Resistance, Evasion, Greater Invisibility etc) also increase the effective ac by +2.
3. Go to the top of the Create a Monster section in the DMG to calculate the defensive CR. This is calculated by going down the table until you reach the row with your warlocks effective hp. Look at the AC column to the left of that, for every 2 points of ac their are different between the AC given in that row and the effective ac of the warlock, move up or down 1 row. This will give you your defensive CR.
4. Now it's the complicated bit, calculating damage. This is calculated by working out the highest amount of damage the warlock can do in 3 rounds of combat when using the average damage values for those attacks/spells then dividing the result by 3. You ALWAYS assume the attack/spell hits, the target(s) fails their saving throw and that any spell with an area of effect (Line, Radius, Cube etc) hits 2 targets. For a warlock, work out how many spellslots they have left if they used up any casting False Life or Mage Armor before combat starts, then work out what the most damaging spells are. At level 5 for example Fireball and Lightning Bolt are the highest damage dealing spells. Once you've burned up your spell slots, Eldritch blast is (usually) the best option damage dealing wise, remember warlocks fire more eldritch blasts at higher levels. Factor in any abilities that might effect the damage of your spells, such as the Agonizing Blast eldritch invocation. It is assumed the warlock gets attacked once per round, so if they have reaction spells like Hellish Rebuke you can factor in those when calculating the damage if necessary.
5. Work out the effective attack bonus. Unless the warlock has an ability that allows them to frequently have advantage on their attacks (Pack Tactics etc), the effective attack bonus will just be the charisma modifier plus proficiency bonus. If it has an ability like pack tactics, you add +1, if it ALWAYS has advantage on their attacks then you add +4. (I don't know why the difference is so huge, this is just what it gives in the DMG)
6. Now we work out the Offensive CR. Go back up to the table and go down the damage row until you reach the calculated average damager per round. Then look to the left of that, for every 2 points of difference between the attack bonus on that row and the effective attack bonus you calculated, go up or down the table by 1 row. Look to the right to find the offensive CR.
7. Add the defensive CR and offensive CR together then divide by 2 and round down. That's your warlocks CR.
As you can see, this method is insanely long and also includes quite a few assumptions as the DMG doesn't explain a lot of things. From experience, a warlocks CR is usually 1/3 of their level if they have very few offensive spells or 2/3 of their level if they have quite a few offensive options. I would advise ignoring the headache inducing method shown above and if you have access to it, use the Sidekick option provided in Tasha's.
I tried this method, and he has a challenge rating of 10 lol
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My homebrew stuff:
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I am an Archfey, but nobody seems to notice.
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So, Baseline Warlock with those spells would have 8 + 9d8 + 10xCon (78 average with +3 to CON for this example), +9 to hit with spells and a DC 17 spell save (assuming maxed CHA by level 10), 2 5th level spell slots and a damaging bonus action attack with the tentacle. Assuming +3 Dex and Studded Leather, that gives AC of 15 and Cold Resistance from subclass. Racial abilities and Saving Throw Proficiencies don't exceed the limits of what would affect CR so lets start
Defensive CR: Hitpoints place you squarely in CR 1 territory, and even if you do the multiplier for cold resistance (which is not recommended as you usually do this when you have multiple resistances and/or more common ones) that is CR 4. Because its only 1 relatively uncommon resistance, we will start at CR 1. By casting Everards Tentacles you gain 10 temp HP which would raise that to CR 2. expected AC for that CR would be 13; because yours is 15 that increases Defensive CR by 1, so the final Defensive CR is CR 3
Offensive CR: The damage table assumes 1) you use your most damaging available attacks on your turn for average damage, 2) combat lasts for 3 rounds, and 3) attack rolls hit and AoE effects affect 2 creatures who fail their saves.. With these, your most damaging Turn choices would be
Average damage for that is 77 or CR 12. Note that Lightning Bolt is (10d6 x 2 = 70) and somewhat comparable to Cone of Cold if used in place of it, and no other spell options you have listed deal close to that amount of damage per round. The expected Save DC and Attack Bonus is 17 and +8 respectively, your 17 / + 9 does not change that number, so Offensive CR is CR 12
Bonuses: If any Feats or Eldritch Invocations would add additional saving throw proficiencies, resistances, or mimic an ability from the DMG Chapter 9 "Monster Features" table found here https://www.dndbeyond.com/sources/dmg/dungeon-masters-workshop#CreatingaMonsterStatBlock then you need to adjust the values above as stated. But the next step is averaging out the Offensive and Defensive CRs (12 + 3) / 2 = 7.5 rounded to CR 7
That Said, this statblock is a bit of a glass cannon (High Offense, Low Defense), so it may not last 3 rounds (or even 1) without some form of boost to AC, Saving Throws, Resistances, or additional HP. That will change defensive CR and total CR, so be aware of that.
I ended up with the same defensive CR as you, but a different offensive CR. Using Evard's Black Tentacles as my first action, also using Tentacle of the Deeps as my bonus action each turn, but used Lightning Bolt as the damage spell for that turn. 60 damage for each of the two targets, times 2 for the two castings, is 240 for the lightning bolts alone. Tentacle of the Deeps does a total of 48 damage, plus the damage from Evard's Black Tentacles (which is 18 for each of the two targets for each of the three rounds, adds up to 108) adding up to a total of 396. 396 divided by 3 is 132. With 20 Charisma and a PB of +4, the attack modifier is +9, which would take the Offensive CR to about 18. 18 + 3 = 21, 21/2 ~= 10.
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Your mistake was using max damage for your spells and attacks instead of Average Damage. the calculation method says to use average damage. The average damage of 10d6 is 35, times 2 for 70 per round used. The average damage of Everards 3d6 is 10, times 2 for 20, and the average damage of the tentacles 2d8 is 9 damage.
The average damage for dice is as follows: 2.5 for a d4, 3.5 for a d6, 4.5 for a d8, 6.5 for a d12, and 10.5 for a d20. The .5 is because there is no 0 on the dice.
ohhh ok thx
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Youre welcome! Its a complicated process, even more so for spellcasters since they have so many attack options, and its especially swingy for NPC's with levels since their hitpoints are always lower than a typical monster's hitpoints
This is exactly why high CR monsters often have legendary saves, and why using PC rules to create NPCs isn't ideal.
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To be precise, how would a (1) Legendary Resistance affect the challenge rating of this creature? Assuming this is using a homebrew magic item to gain this legendary resistance.
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For your warlock, each use of legendary resistance it has per day increases it's effective hitpoints by 20hp per use
With this addition, the overall CR would change to be 9. Thanks!
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Legendary resistance alone doesn't exactly fix the glass cannon issue, I just brought it up as an obvious example. The larger point is that encounters with opponents that are heavy on offense and light on defense can be a bit volatile - if the PCs get lucky it'll be over really quickly and the encounter will be an anticlimax, if they're unlucky the whole thing can get out of hand fast. Nonetheless, you do you - it's your campaign, you know best.
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