Thought you saw the last of Gara'jal in Mogushan Vaults, did you?
Yeah, so did I. Mogu'shan Vaults was merely a setback!
The Council of Elders is the third encounter in the Throne of Thunder. I surmise it's the whole leadership of the Troll empire right there looking to stop your raid team from progressing any further into the compound.
As you can guess, this encounter is the first council-type encounter of the raid. They'll have to handle the various abilities and spells that the Council is dishing out while slowly whittling them all down. They don't have a shared pool so you do you have your work cut out for you. Now there are going to be a ton of things going on but your raid should be able to pick up on the different abilities without too much of a hassle.