Unseen are invisible mobs that haunt Raven Hill in Duskwood. They are neutral unless attacked, but since they are invisible to the naked eye most players are totally unaware of their existence. Warlocks who can cast Detect Invisibility, or other players using various means to detect invisible mobs, can see them. In addition, they do not become visible when provoked.
Since they are invisible and neutral, they are generally not a threat to players. However, to any class with an AoE spell they can be extremely dangerous as they are level 49-51 and Duskwood is an 18-30 zone, so any players who encounter them are very likely to be a much lower level.
Flares and tracking cannot detect Unseen. The Warlock's detect invisibility spell allows anyone to see them, but Mages who are invisible can also see them.
Technically, Unseen are Easter eggs.