The final campaign – “Even Angels Cry” [7th campaign – 1st mission].

in general, it appears on the eighth day after capturing the fortress where the souls of angels are imprisoned. But it can also appear on the third day if you conquer the last angel castle and destroy all their heroes.

Yeah... it turns out to be a tedious mission, not a fun one :( Because the fortress and all the angel cities are captured by Anastasia with her starting army...
So, I'll have to artificially delay capturing the fortress in order to build up an army. That's a shame :(