Maybe her own links to the spirit world were used to deceive his?
"Varimathras taunts Horde
Varimathras: So, she found me at last. Sent her underlings to finish the job.
Varimathras: Tell me, when she seized your throne of hides and bones, was your allegiance forced? No... I'd wager you surrendered it willingly... or were convinced you did.
Varimathras: It matters not. You are blind to the darkness in your midst."