The hydra Chimaeron is the third of the guardians of Blackwing Descent's Vault of the Shadowflame.
The tanks need to be aware of how to manage Chimaeron. He will periodically use Double Attack which will hit twice for very high damage. Since there are two strikes Bile-o-Tron's death immunity will not save the tank. This means that the tank that is taking the Double Attack needs to remain at high hit points throughout the fight.Cataclysm Raid Guide - Chimaeron.