I didn't like the thought of keeping track of instant-speed trickery of removing creature cards from graveyards to beef up this card, so I made the sorcery-speed restriction.
Fun fact: The only "Mass of" creature is Mass of Ghouls. Now, in a somewhat poetic move, here's a Mass of Souls.
Man, the original design of producing Zombie tokens just doesn't match the art here. Sure, you can say that the spirits of the dead left the bodies; and, thus, the bodies can become zombies - but then why doesn't the art focus on the zombies instead of these spirit-looking things? Answer: something went wrong during Mercadian Masques development in regards to the original card. I fixed it here by making this a Spirit card!