My guess is mob spawns for last four new mobs to game (there are six types total).

*Skeletons (watchers tombs and krksne mines lvl3)
*Aiger (kraken maps)
*Yetis (nordr maps)
*and whatever the shuyal mobs are (i guess shadow mancers).

We know from screen shots that skratch spawns differently each time. I assume each boss replaces a random mob. Happy hunting.