A super unique is a monster with a special name that spawns every game. In normal difficulty they always have the same Unique Monster Mods. One thing that can make a superunique particularly difficult is the fact they can get more unique monster mods than a normal randomly named unique common monster. In nightmare difficulty they will have one extra random unique monster mod. In hell difficulty they will have two extra. Some super uniques are just common monsters such as Bishibosh and Corpsefire. The rest on the other hand are bosses of varying calibers ranging from mini-boss to the well known act bosses.