Necropolis – “Dawn from All Sides” [Campaign 2, Mission 3]

I defeated him easily with fifty Fate Weavers – I didn't prolong the game, I just explored the dungeon.

So, is capturing the underground castle mandatory? The map is indeed quite challenging. Moreover (until the latest patch), whenever I defeated one of the demons in the dungeon, each of my turns was followed by some kind of "lightning" (before the start of each turn), which reduced one of the tiles with my creatures to zero.
Have you encountered this? Or is it a bug?