The Skeleton Legion was the name given to the primary army of the Cult of Skulls. Unlike any other army known on Terfall, the legion was solely comprised of undead with only the cult's Necromancers as the controlling force.
The legion played a primary part in the cult's power during the Necromancer War and it's control over Jaqua. However, when the cult lost its leader, Mardrake Transfer and his Death Guardian Morgrim Katanova, the cult could no longer maintain the magic needed for the legion and it all together dispersed.
The Legion was formed from Mardrake Transfer and the rogue mages turned necromancers within the eternal graveyard's using necromancy to summon every fallen solider that existed in that deathly valley.