The Lamasus were a failed experiment by the wizards to create a superior breed of beastmen, by the unnatural fusion of a human being (usually a slave or criminal) and a magical creature (in this particular case, a Manticore). The resulting creature was too unstable and was literally short-lived, but they still proved useful to the Necromancers. The Necromancers rose their dead bodies and infected them with pestilence. The Lamasus function as plague vector, coupled with the creature’s physical might, and its mobility make them perfect front-line troops in the Undead armies.