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

there are only five squads of demons that pass through the portal during the game
a couple of powerful armies and three with initial troops
by the time you break through to the assassins, all five armies will already be running around the map

Added 1 minute ago

I killed the assassins and returned to the ghost. He said that I would be able to enter one of the portals. In one, his daughter is imprisoned, in another, there are various goodies, and in the third, there are 3 level-up items.

so, only one of the three portals can be used as a reward, or did you save and go through them?

The daughter is from which castle, Human or Necropolis?