Bosses

Bosses are stronger monsters that have a lot of life points, do massive amounts of damage, and can only be summoned by some special item. You get these special items by collecting a certain amount of normal items (for example to get the "suspicious looking eye" you need 10 lens'), you then take the items to a Demon Alter found in or near corrupted areas to craft the special items needed to summon bosses. Killing these monsters will take a lot of time, but once done, you get a lot of loot, such as armor and items unobtainable by other means. Killing bosses alone is hard, so it's easier to fight it with someone else.