Hydras get +20 hack for each new head
Hydras get +20% attack for each new head
Each new head gives a diminishing attack bonus; i.e. the 2nd head gives 20 hack, the third 10 and so on.

Being a little bored this morning, I decided to "bottom this out" in the editor - and I was quite surprised by the results!

My main tests involved using a hydra to slaughter varying amounts of deer, before attacking a war elephant (chosen due to the fact that it has lots of hitpoints and 99% crush armour, so the hydra's crush damage can be effectively negated).

I also repeated the tests against hippikons to ensure that the target unit does not impact the attack dished out by the hydra (it doesn't).

I further repeated the tests by slaughtering varying numbers of axemen rather than deers to ensure that the unit being killed doesn't affect the hydra's increasing attack.

Sure enough, the hydra gains a new head every three kills - and each kill impacts the hydra's attack in the same way... with the strange exception that if you kill deer etc. to increase a hydra's attack, the first deer you kill doesn't change anything! So you need to kill 4 deer for a second head, and 7 deer for a third head... but only 3 axemen for a 2nd head and 6 axemen to gain the third.

I'm not sure why the first deer doesn't register on the attack bonus scale, but the first axeman did - however I'll display the results for the axemen as this is a minor issue that shouldn't confuse things too much.

I also timed the numbers of attacks that the hydra delivers per second, and according to both the in-game clock and my stop-watch, it attacks approximately every 1.3 seconds - so this had to be factored in.

My results were as follows:

Killing deer for extra heads, and then testing vs. a war elephant with 25% hack armour:

Kills--Heads--HP removed/atk--Damage/atk--Dmg per sec

0------1------24--------------32-----------25
1------1------28--------------37.3---------29
2------1------31.5------------42-----------32
3------2------35.5------------47.3---------36
4------2------39--------------52-----------40
5------2------43--------------57.3---------44
6------3------46.5------------62-----------48
7------3------50.3------------67.1---------52
8------3------54--------------72-----------55
9------4------57.8------------77.1---------59
10-----4------61.5------------82-----------63
11-----4------65.5------------87.3---------67
12-----5------69--------------92-----------71
13-----5------69--------------92-----------71

So there are some surprising conclusions to be drawn!

1) A hydra appears to have a base hack attack for 25 hack per second, rather than the stated 20.

2) A hydra will grow a new head every three kills (nothing surprising here) although if it is killing deer etc. the first kill doesn't count for anything. This aside, all units killed by the hydra seem to impact its attack in the same way.

3) The increased attack that a hydra receives upon killing is incremental per unit killed, rather than per head received.

4) A hydra's hack damage increases by approximately 4 hack per second for each unit that it kills, up to a maximum of 12 units which is c. 71 hack damage.