All players and monsters in Midnight Sun have an 'alignment' stat which measures how good or evil the character or monster is. Similar to how stat names have descriptors, alignment has categories as well. They are:
Saintly -> Virtuous -> Good -> Nice -> Neutral <- Nasty <- Evil <- Malevolent <- Demonic
Every time you kill a monster your alignment will shift a small amount. The more evil the monster you killed the more alignment you gain. The more good the monster you killed, the more alignment you lose. Some guilds require you to have your alignment at certain levels to access all of your powers (Priests require you to be as Good as possible, while Druids require you to be as close to Neutral as possible etc. Several guilds do not care which alignment you have at all.)
Many items in the game also have restrictions to use them based on your alignment, while others have increased or decreased effects based on your alignment, or the alignment of enemies you fight.