Specifically, at which point (quest, cinematic, dungeon text, etc) did we learn Gul'dan was going to use Illidan's body as a host for Sargeras' spirit? Because I heard speculation about it back when Legion was released but never saw anything in-game confirming it. First time I saw it in-game was in 7.1 with Xe'ra saying "If Sargeras truly plans to use Illidan's body as a vessel for his own return..." like it's already been stated before.
So what did I miss? When did we first learn of this in-game?