Assuming each turn the hydra takes exactly 25 damage, and 0 fire damage is done to it throughout the fight, what is the maximum number of heads a hydra could get before it dies?

If my math is correct, you should be able to chop 11 heads before it dies. ( with the Hydra's average hit points)

My math says 31 heads.
For each head you chop of, the hydra heals 20 hp (10 per new head). That is a net of 5 dammage per head. After 30 heads the hydra is down to 22 hp and the 31'st head you chop will finnish the hydra before it has time to regrow new heads

35 max heads for 172hp before dying or 55 for max hp.

YW.

Challenge Rating 8 my ass

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.

You can make the argument that you CAN’T kill it without fire if you keep chopping off heads.  It’s not clarified, but it’s possible it will regrow two heads after a killing blow, leading it to come back to life at 20hp.  If this is the case, you need to do sub-20 dmg to avoid killing a head, or use fire.

 Personally, I would ignore its actual health and make it a flat out requirement that you need fire to kill it. Sticking to mythology the only way to kill this thing it to kill all its heads, therefore it would be a potential limitless encounter with infinite heads each requiring 25hp worth of damage to kill one and then cauterising the wound with fire to ensure it doesn't regrow.   

You could even run an encounter and have each head make its own initiative roll, with individual HP per head counting them as seperate creatures, but having them all move on the hydras bodys turn. This would benefit the players by having the damage recieved from the Hydra more spreadout across a round, but also benefit the Hydra by not allowing it to simply be nuked down before it even gets a turn in combat if it had a poor initiative roll. 

Hail Hydra

Could it not be hundreds of heads? If you got it close to death, then healed it; rinse and repeat? :P

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

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.  You could circumvent the possible 30 heads by dealing 24 damage or fewer each round, thus cutting every one of its heads with mortal bleeding wounds but not actually severing any heads, or dealing a lot more than 25 damage per round, causing about 11 heads to be present when it dies.  And, of course, fire damage.  Any primary magic user has at least one fire based attack spell.  At that point, even at max hp you can deal 125 damage to it and actually kill it.  

It's hard to control the damage to be sub 25 and be close to 24... I mean. Even if that were true and you don't cut off the heads, dealing exactly 24 per turn, that's 7 turns with 5 attacks each turn at +8 and 1d10 + 5 damage... lol. You gotta slice those heads, bruh hahah.

Not to mention, if you start dealing like...16... and they you're hesitant to deal another 9 to it so no one else attacks it, that's just another turn with 5 heads.. lol

Love these bad boys...

I have to F***ing fight this when I start my world with my friends lol...
Luckily I am using THORAKK THE MOUNTAIN DWARF BARBARIAN (He is OP as)

Quote from BennyDM >>

It's hard to control the damage to be sub 25 and be close to 24... I mean. Even if that were true and you don't cut off the heads, dealing exactly 24 per turn, that's 7 turns with 5 attacks each turn at +8 and 1d10 + 5 damage... lol. You gotta slice those heads, bruh hahah.

Not to mention, if you start dealing like...16... and they you're hesitant to deal another 9 to it so no one else attacks it, that's just another turn with 5 heads.. lol

Love these bad boys...

Something important to note is that it specifies "25 damage in one turn", not round. That means that the damage doesn't stack. Your fighter might deal 25 damage in one turn if they action surge with GWM and GWF and roll well, but the 25 has to come solely from them. If they roll low on damage and only deal 16, then the Druid doesn't have to worry about only dealing 9; the counter starts over because it's a new turn.

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

Dunno as noted above needs 25 dmg in a round

i can see this thing going sideways if nobody hits it with fire

What if the characters specifically say they are NOT attacking the heads?

Also this thing is EASY to beat with a flametongue weapon.

also, imagine the hydra, but replace fire damage with cold damage and give it the Fire Absorption ability, kind of like the lightning absorption ability of the shambaling mound, but with fire, then it becomes a lot harder to beat with a flame tongue weapon.

It does not say  the damage HAS to be at the heads. Just says when it TAKES 25 damage. It's a little odd, but that's how it's written. So, where you hit doesn't matter. Just if you do 25 damage a turn. 

Ok, this beast is insane!

TPKed my entire group. Easily!

It won't be an infinite encounter. "If all its heads die, it dies". If you reduce its hp to 0, it dies because it only regrows heads and regains hp at the end of its turn, provided it has not taken fire damage.

Calm down. Hahaha.