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

Help! I've already sacrificed all my Necromancers at the altars, but I can't find the passage to the Assassins. I went to Sanctuary, and all my monsters are undead and upgraded, but I still can't defeat this demonic army, and they have a strong catapult. Could it be because I didn't fulfill the ghost's request?

Added after 13 hours and 32 minutes
Help!