[Admiral_Aorta]

Originally Posted by Valerius

The eater of the dead has multiple forms. Its initial form is unit 994 and the game checks to see if that unit exists. If it does, then casting the spell again will fail. Once the eater grows in power and shifts to unit 995 then you can successfully cast the spell again since the game will not find that unit 994 already exists.

I seem to recall that if you get the new eater of the dead to its final form it will replace the old one, because there can only be one at a time. Not quite sure though.