Undead are beings that are powered souls of the dead that have have come back to the Mortal Realm through Death Energy. They can only exist with its unholy life. Some undead come about from curses or by being reanimated by Necromancers .
Undead where created by Theda as his verison of Demonkin. These vile and twisted creators where his playthings and he enjoyed to use them against the living in his idea of restoring order when life grew to strong. Undead have had many forms since its first creation as Theda loves to bestow them as gifts to his friends. However Skeletons are generally weaker then most because they require life essence to sustain themselves in the Mortal Realm or else they are sent back to the Underworld.