A Relic Reborn: the Hydra was added in patch 2.0 of the A Realm Reborn expansion.
A Relic Reborn: the Hydra becomes available at level 50 as part of the “A Relic Reborn” quest, which is given by Gerolt – North Shroud (X:30, Y:20).
The trial has a minimum ilevel of 52 to enter and rewards you with:
Allagan Tomestone of Poetics: 10