Tyene's death is deeply disturbing, even though we didn't love her character. Remember when she poisoned Bronn in the dungeons in Dorne, and only gave him the anecdote when he told her she was the most beautiful woman he's ever seen?
Time of death: Episode three, "The Queen's Justice."
Cause of death: Cersei poisons Tyene, who she poisons the same way Ellaria poisoned Myrcella: with a kiss and a poison called "The Long Farewell." So Ellaria has to watch her own daughter die a brutal death, and in another sick twist, decompose before her own eyes, and there's nothing she can do about it.