how come it can not have the regenerating heads +extra head (unless mortal head)

How does something that prevents healing affect it when it regrows heads? Chill Touch, for example, makes it so you that it "can't regain hit points". Does that mean it grows the heads, but does not regain the 20hp?

Quote from RandomToon >>

How does something that prevents healing affect it when it regrows heads? Chill Touch, for example, makes it so you that it "can't regain hit points". Does that mean it grows the heads, but does not regain the 20hp?

 Yeah, that sounds right to me. The regrowing part only specifies whether or not it took fire damage, so no fire damage = it still grows the heads. But doesn't regain any HP. At least, that's how I would play it.

Easiest way to kill an Hydra:

having a 6th lev. Wizard with INT maxed out, that knows how to choose his spells properly...

https://www.dndbeyond.com/spells/slow

Now the Hydra only does 1 attack per round, no reactions, has 13AC and disadvantage against the previously mentioned Wizards FIREballs spells.

You're welcome.

You only need a 5th level wizard to cast slow.

Quote from Wulfwithin >>

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.

 If it isn't outright killed and the DM is not using instant-death for slain enemies, it will still have a turn where it will make a death saving throw and then regrow two heads to net itself 20hp.  With many creatures that regenerate (trolls, some undead, etc.) simply reducing a creature to 0 HP is not the end of the fight.  

But u need a 6th level if you wanna cast the beforesaid Fireball at it...

You learn two new wizard spells whenever you level up, so you could learn both slow and fireball when you get to 5th level.

Would Chill Touch stop the head growth?

Hydras can have an infinite number of heads

With a maximum of 31 heads in a combat scenario, it would be able to make 31 bite attacks per turn. Assuming all attacks hit, it would be able to do an average of 325,5 damage per turn, with a maximum of 465 damage. This would be more than double that of a Tarrasque (excluding legendary attacks).

And of course an evil monster-keeper can keep taking off heads and healing it, giving it infinite heads. I wonder what a 1000-headed Hydra would look like.

I think they hypothetically could have any large amount of heads, but only if someone is trying to do that. If you’re trying to actually kill it, I think the numbers people have been saying are pretty accurate

Can this only have 5 heads at a time or can it have more than that?

I’d have to up the CR but if I ran this in a game. I’d give each head its own turn. And each one would have to be killed with fire or have the stump burned before its next turn or two heads grow back. And doing a nova damage and killing all the heads would not put right kill it unless fire were still used. The myth of these things was that the fire cauterized the neck stopping the heads from growing back. Just lame otherwise. 

Quote from PepTheRed >>

Easiest way to kill an Hydra:

having a 6th lev. Wizard with INT maxed out, that knows how to choose his spells properly...

https://www.dndbeyond.com/spells/slow

Now the Hydra only does 1 attack per round, no reactions, has 13AC and disadvantage against the previously mentioned Wizards FIREballs spells.

You're welcome.

 Enter Hydra number 2

Quote from Qoray >>

Ok, this beast is insane!

TPKed my entire group. Easily!

How many player did you have and at what level? Im considering having my group fight a hydra during our next session. I want it to be challenging but I don't wish to have them all killed.

Could a hydra bite off its own heads in a pinch? It regenerates heads at the end of its turn. I’d probably make it so that it has to use two heads to take off one, based on how much damage each bite does. It would forgo two bite attacks to make it better off in the next round. I’m not sure if it would think that strategically with an intelligence of 2. 

for a team with a mage that knows any fire attacks yes, but if no one in your party has any way to deal fire damage it seems pretty difficult. but chances are someone will have  fire damage.

This is why I took the Fiend patron for Warlock...

By my calculations, if you get rid of one head per round, it should be able to have 39 heads before dying, if you use the provided hit points above. You do 5 damage per turn, so 172/5 is 39. The hydra will have 2 hit points left.