Jynnx, i'll show you exactly what you need to do(it still doesn't happen so it's a bug)
-You need to get the quest AND find/kill the elite boss before the Ch.4 Boss
-You have to click those "flashy" whispers 10 times, no more, no less
-After you 10th click a mensage'll appear on the screen, when it happens you have to go the elite boss place(that is empty) to see a cinematic where the elite boss appears and then fight it.
Things you don't have to(or mustn't) do to found the elite boss
-Do another quests
-Kill the main boss
-Kill monsters or mini-bosses
If you did all the things i told you above, tell me if the mensage appeared or not. Maybe you lack attention, maybe it's a bug.