First, I just want to say, if this was supposed to go in the aquila category, I'm sorry.

Anyhow, I'm level 67, and just finished fighting the hydra 18 times (The first 6 were for the quest, the rest were for science). I may have discovered something about the hydra, I believe that if the ebon head is dealt 25% damage during the first or second round, it will be invulnerable the next time it appears. This was tested by me and my friend 18 times, the only times Ebon came back invulnerable, were the times we dealt 25% damage within the first 2 rounds.
If you could please test this information yourself, it would be appreciated.

Thanks,

Dark Ryan Ashburn, 68
Dark Blake Ashburn, 34
Dark Destiny Ashburn, 26