Note that Magus of the Coffers is too slow for the ultra-fast combo deck you're hoping to build. I'm also unsure of what Umbral Mantle is supposed to do.
EDIT: Just realized that you can go infinite with Magus of the Coffers and Umbral Mantle. Even so, it's too slow for what you're wanting to accomplish.
My EDH decks:
Sliver Overlord ()