Skeletal Summoners are Undead.
These skeleton shaman spawn with packs of skeletons. They are larger than regular skeletons, and have glowing purple/pink orbs around their upper bodies. These may be shields, decoration, or an indication what type of elemental damage they deal. During battle these summoners stay to the rear of their foot soldiers, where they keep busy summoning fresh skeletons, or hurling purple projectiles at any players who stray into range.
Their summoning animation is interesting; in the WWI 2008 movie they were seen shooting a purple laser beam from each eye. The beams traveled some distance, then converged and dove into the ground, causing a new skeleton to rise up a second later.