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AION: The Tower of Eternity is still under development by NCsoft, and any information related to the game is subject to change with the release of more information.
A Mobile Object, or Mob, is a term used to refer to an NPC monster in an MMORPG.
Mob is also commonly used to describe a group of monsters that may be linked to eachother.
Some mobs will automatically come to attack players when that player comes within a certain range of them (for more on this see aggro).
Mobs will usually drop some kind of loot upon death, and the player(s) that killed them also gain some XP.