So I want to see your posts, do you agree this whisper refers to Sylvanas? If not, who does it refer to and what does it mean to drown them?
Perhaps deep in Torghast there is a well. It's surface blazes bright, but it masks shadows below. After we do the impossible and defeat the Jailer, we drop a dying Sylvanas in it so we can rid of her. Any other method would result in her return. This well has a connection to the Void. In Sylvanas's final moments, as she ponders her life... Her fall to Arthas which she though was the end, her suicide at Ice Crown Citadel, she thought was the end, and now her defeat in the Shadowlands, she knows this time, it's the end.
The Void reaches out like they did to Queen Azshara. Sylvanas Makes one final deal opening a rift like she did between Azeroth and the Shadowlands. She opens a rift between the Void and the Shadowlands. We are unsure of Sylvanas's fate in the aftermath.
The Voids forces come through this well, overwhelming the Shadowlands and seek the titan sole on Azeroth next. We barely manage to escape the Shadowlands. It looks grim, we are waiting for Voids inevitable invasion of Azeroth.
The void finds a prisoner in the Maw they seek mold into their new champion, his name.....is Arthas.