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

You need to place Ornella and three Necromancers bought in the tavern at the ritual sites near the four cities. They are hard to miss.

By the way, does anyone know what happens if you don't send "assassins" against Demonessa, but just kill her? Will you still lose Ornella when she burns in the fire (at Demonessa's location)? And what reward will you get from the Spirit of the slain Mayor of Holyflame? ... I just finished the game, opening the altars myself, using four heroes...
P.S. The Necromancer campaign is always the easiest... I played on the highest difficulty (Hero), leveling up both Arantir (level 28) and Ornella (level 29).
Ornella only used Vampires and by the end of the game, she had almost 500 of them (I tried to only raise them and only un-upgraded ones, and together with the Necromancer set that reduces the cost of Dark Magic, while Arantir used Mummy Lords and Death Knights... for the final battle, I gathered everything together, although only the Vampires could have defeated everyone.
There, the defense was Hero with stats 30-24-18-16 and an army: 52 Archdevils, 89 Cave Lords, about a hundred Hellhounds, 197 Demoness, 420 Hounds, and 500-600 Demons - all, of course, were fully upgraded.
And I had 42 Spectral Dragons, 60 Undead, 200 Archliches, 500 Vampires, 370 Ghosts, 800 Zombies, and 1200 Skeletons