Our freaking DM is making our party fight this at level 3, currently we have 1 down, 1 at 4 hp, another at max, then I'm at 64/85 hp.

I counted around 51 heads chopped off before the killing blow. (Max. HP)

these things are kinda scary lowkey

Hydra is such a versatile enemy. You could have mini hydras that start with just one head (obviously lower challenge rating) so really only 25 hp but could turn into a boss if handled poorly. You could also have elder hydras that start with a ton of heads (and extra hp to compensate)

31 is correct.  Although about 8 rounds before that the hydra is doing 100 damage to a single target. that target would need 160hp.  With 18ac would need healers to cover 1837 damage in the 26 rounds  and heal for 132 the next to last round. 

Just ran it with a group of 5 level 8’s. No fire, no team buffs except bard inspiration. . 1 tank 19ac. 4 rounds. Died with 9 heads. Tank bloody.     Hydra Started with 150% health, 258hp.  Not A challenge for this group. 

Quote from ItsNotAChest >>

Quote from Agathon >>

is it me, or does this seems rather....underwhelming?

 Agreed, I feel like it should have legendary actions

 And lair.  Lvl 7 and up

Theoretically, who would win in a fight between a hydra and a T-Rex. On one hand the hydra has it's doubling heads and becomes more damaging over time but on the other hand Rexy's bite is absolutely devastating 

Use awaken on it CR over 9000

Awaken the hydra

Why were you looking at stat block in the middle of a fight? 

"Cut off one head, two more shall take its place.  Hail Hydra!"

-Red Skull

I'm trying to create a different version of this and post it,but your bots are saying it's to similar to the original. I've completely changed it, I only based it off of the hydra because I needed the head regeneration to be similar to the Hydra. 

Quote from KaptainKittens >>

So many heads and such low intelligence, any brains in there?

If one of these was tamed as a pet in some way, is it safe to assume if the tamer was aware of the properties, it would try and give the hydra as many heads as possible without killing it? Might require a heck of a lot more food considering their tendency to clear territories and then eat it's own heads.

 Wait, they eat their own heads?Nifty

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. 

Now this is a monster that could kill a whole decent sized town. Many tough monsters can still be taken down by town defenders if they rally enough people and make a bare minimum effort to fight strategically, (essentially mass crossbows, barricades/walls/cover, and try not to die too quickly) especially if they have a couple local tough guys to help like the head guard, clerics or mages, retired veterans, gangsters protecting their home turf. To survive in a DnD setting a town has to be able to survive the occasional orcs/gnolls/goblinoids, rampaging chimera or the like. But a town (or party) that isn't prepared can be unexpectedly swept aside by a hydra. Though if you're going off of just the rules, mass guards/militia/whatever won't even destroy the heads and won't trigger it's healing making them damn good at killing hydras.... Though the hydra could also be damn good at killing THEM if they're stupid enough to let it get close to a group of them. 

What's stopping the hydra from farming for heads by attacking itself? Aside from its low intelligence.

51 heads. It regrows two heads for each lost, and gains 10 HP per head. That means reach turn, it gains a net off one head, and -5 HP. Maximum HP for a hydra is 255. Since the last 25 damage will take kill it, we dived 230 by 5, getting 46. Therefore, it will gain 46 heads. Added to the original 5, you get 51.