Quote from Rippington >>

I only see one possible way to one-shot this thing: Fireball

 As always fireball is the answer.

I used a Hydra after watching a Video from a small Youtube channel called "Hidden Nerdy Side" which suggested giving the Hydra long necks and make the encounter take place in a pool of water, where the Party is trapped on a small Plattform. 

So I had them on a Boar crossing a Cave Lake when five "sea serpents" raised out of the water and attacked. It was a great fight, especially since noone could meta game "Wizard just use firebolt every time so it doesn't regenerate!" because as far as they knew, the encounter was just alot of small creatures with pretty weak attacks each. And since the Pary was in the center, the Wizard couldn't just fireball the whole thing. 

I also added a mechanic where one head could go underwater, therefore skipping its attack for 1 turn, and then come back the next turn and use the Tidal Wave spell by spitting water on the boat. 

I'm making a Hell Hydra monster for my players, can't wait to see them try and hit it with fire only to heal it.

How do you tame a Hydra because it says unaligned?

If you want a cool magical variant of this monster check out the dracohydra found in Fizban's.

I'm late to the party, but, put 3 of these against a group of 3 level 15 adventurers with excellent offense and defense resulting in a maximum of 7 heads and 1 almost dead paladin.  The rest flew away to do their work at range.  If the party almost kills the hydra without fire damage, the next turn it is around is a horrific experience.  "I'm going to roll 13 attacks against you with advantage from flanking. "WHAT?! WHY?!" "There are 2 hydra on opposite sides of you." "Well I want to fly away." "OK." The rest of the party: "NO DONT FLY AWAY THEY WILL KILL YOU."  

Crew just faced off w/a hydra. My lvl 8 Tabaxi Druid (Circle of Spoors) managed to kill off 7 of 8 heads with Blight, then finished off the final head with the Chill Touch cantrip.

Lucky Friday the 13th INDEED!

=^.^=

Challenge rating ate my ass too

if you mean as a player that true polymorphed into a hydra as long as it's healed 5hp per turn it can infinitely gain heads but as an opponent you divide 175 by 5 which is 15 so 15x2 is 30 so 35 heads in total

You have the highlighted part right. It gains 2 heads and 10 hit points when you cut off a head. The head being regrown splits into 2 heads its not counting each new head in the healing. If you cut off both new heads it gets 4 heads and 20 hit points and so on.

Imagine this scenario please:

1v1 Barb/fighter VS Hydra

turn 1: PC attack 100 dmg with some fire dmg- hydra lose 1 head

turn 2: Hydra attack - no regen cause fire

turn 3: PC attack 100 dmg no fire dmg - hydra lose head

turn4: does it even happen? At this point hydra is at 0 hp so no action but it can regrow 2 heads healing 20 hp. Back to 5 heads

turn 5: PC attack 50 dmg no fire - hydra again lose 1 head (down to 4) and at 0hp?

How does it play?

Can hydra drop to 0hp with heads still attached to it?

Please help I am confused.

36 heads!

172-100+80 (-4+8)

152-150+120 (-6+12)

122-100+80 (-4+8)

102-100+80 (-4+8)

82-75+60 (-3+6)

67-50+40 (-2+4)

57-50+40 (-2+4)

47-25+20 (-1+2)

42-25+20 (-1+2)

37-25+20 (-1+2)

32-25+20 (-1+2)

27-25+20 (-1+2)

22hp (35+1 heads)

Just fly out of reach and bomb it with fireball, what's it gonna do to you?

The hydra is one of my favorite monsters in dungeons and dragons and I would love for the wizards of the coast party to make a new variant, preferably one that could be found in the forest and mountains I am currently working on an idea for one.

Good question I have no clue 

Quote from ShaydyCharacter >>

Quote from Chemicazo >>

Remember, all of what you are hypothesizing about is if and only if an entire team of adventurers deals exactly 25 damage to it each round.  Even if this is an encounter for sixth level characters, 25 damage between a tank, healer, magic user and damage dealer is pretty weak sauce.   

 It's per turn, not round. The damage isn't shared between players. 

51 if you roll perfectly on the hp

Awesome mid lvl polymorph

WTF do you have THAT many hp at this lv wtf?

Quote from UnknownKnight06 >>

I've always wanted to use a hydra, I even have two minis for it, but my issue is that since fire damage is so common, and even a torch as an improvised weapon could do at least 1 point of fire damage, it seems kinda wasteful. Like almost every party has at least 1 fire based dealer. I know in my current group have have at least two for sure. So I'm thinking I'm going to take the fire damage part out, and replace it with another kind... keep the party guessing and on their toes, but also have it be something they could use. I'm thinking acid, but I'm not sure? Any thoughts?

I'm thinking of running one of these in an encounter; my plan is to treat each head separately for the fire damage caveat, so instead of 1 point of fire damage preventing any regeneration of heads, the fire damage only prevents a specific head from regenerating, i.e- any head lopped off with only slashing damage etc. will grow back. If you look at the legendary hydra Polukranos from the Mythic Odysseys of Theros, it actually has slightly different wording for its head regeneration which means that it takes 40 damage for it to lose a head, and it must take 40+ fire damage to prevent regeneration, so I might alternatively run it as-is but with 25 fire damage as the requirement.

Otherwise I think the fire damage caveat is too easy to trigger and makes the monster far too weak, same as a troll, though I guess you can have the creature focus on eliminating the fire damage dealer(s) or actively avoid them, but that usually feels lame in a fight, especially if a creature can be making 5+ attacks against a single player that way (I feel it's more fun to have multiple players fighting multiple heads as groups).

Update: The wording I decided to go with was to replace ", unless it has taken fire damage since its last turn." with ". Slain heads that took 10 or more fire damage on the turn do not regrow.", so it needs to be more than just a gentle caress with a torch. Something like oil (flask) which can amplify weaker fire damage is recommended, or elemental bane if you're a high enough level.

I think the other way to make this encounter tough is to choose the environment carefully; I'm thinking of a cistern in a sewer, so it has water to dive under to evade fire damage and reposition itself, with the chamber also making it hard for the players to evade the monster if they can't get out. I'll probably have a tunnel be usable as an escape if the players want to flee, but be narrow enough that they can't use it as a defensive location (it will limit the number of players who can attack, and focus the hydra's attacks upon those players if it pursues).

This way it can still be a tactical challenge despite having only Intelligence 2, as simply by acting reasonably for an instinctual predator it still has a lot of options (fire bad, go underwater, enemies in tunnel, give chase etc.). If you wanted to make the encounter more deadly you could have the players fall into the cistern (cave or whatever) and the only way out be locked or blocked by something you can't reasonably do while fighting the monster, or would be risky to do, e.g- maybe a dead body has the key to a locked gate? You could grab it, or try to pick the lock, but you're spending turns doing that during a fight unless you defeat it first, could make for a very tense encounter.

I love the idea of having the hydra appear as just giant snakes in the water though, so the players don't realise what they're fighting straight away…

Lastly, regarding the "how many heads" debate, I interpret the everlasting hunger line "a hydra’s hunger is so great that if it can’t feed, it might turn against itself, its heads attacking each other as the creature eats itself alive" as meaning that five heads is the "optimal" number of a hydra, any more and it may start eaten its own heads (which if it's starving presumably don't regenerate), with five being a manageable number in most environment, as more than this will mean literally more mouths to feed, and more likely to scour the area clean faster than it can find more food.