Within the game, there are several 'boss' characters. These bosses are just more powerful enemies, and may show no distinct visual difference than other enemies. You will know you are about to fight one a boss when a sigil forms from the map side that you came from. The room containing the boss also shows as highlighted in pink on your map. The sigil will only be removed once the boss has been defeated. There are several other enemies surrounding the boss as well, and prove a hinderance in combating the boss. In some cases, all enemies together are considered a boss, and the sigil will not be removed until all have been defeated, as in the North Forest goblin quest area.