"The Will of Ashan" - "Heart of Darkness" [2nd campaign, 5th mission]

I placed 4 heroes on 4 altars, but nothing happens... I want to get to the trees... And if I send assassins to the demonesses, it immediately says that the mission has failed... It's silly, of course, but I want the gift from the ghost)) 2 free levels for Arantir.
What should I do? How do I destroy the altars?