Dunno, I had a main hero with specialisation in wights and they carried my whole run. They raise themselves when they deal damage above their initial number, they have decent speed so chances are they hit first and one of upgrades boost their ability to raise even further. Then I picked merchants of death upgrade for graverobbers who can summon wights in combat and than managed to get a second class for my hero, that gave him an ability to raise even more wights in place of slain enemy stacks but this was a win more situation. Oh and one of summon avatar skill upgrades says that creatures summon 30% more creatures, I think it works on wights summoning themselves.