Mistress Sassz’ine is the fifth boss of the Tomb of Sargeras raid instance and a devoted fan of ocean fauna. And a representative of it as well, actually.
According to the lore, Sassz’ine for quite some time has been having fun learning how to bind as many weird deep water creatures to her wicked will as possible. Well, you got it right, she is called mistress for a reason: at least three monstrous sea denizens fell victims to her irresistibility, which is a destructive example of spinelessness in all senses.
Tactic-wise, you will have to deal with hundreds of adds, walls of tornadoes, ink pools, gigantic waves, devouring maws and whatnot – all this grandeur divided into two (raid finder difficulty) or three phases. You will have no choice but to move really a lot here and in this regard the encounter is, probably, one of the most exciting in the entire raid.