This looks good and fun. I'd run 24 lands, you don't have any 1 drops, and it just seems like the right number. Mutavaults and Pontiff of Blight also strongly encourages a higher mana base.

First thing I notice is this deck needs more removal. That's good, because I see room to cut.
-Font of Return. 3 cards for one is good, yet also slightly overwhelming. You'll never need two. And the high cost to do so just might not be worth it in the first pleace. Honestly I'd run Whip of Erebos over this any day. That'd be much more effective too with your Gray Merchants, Lifebane Zombies, Liliana's Reaver, and Blood Scrivener (by not filling hand).

-Pontiff of blight. First off, the dude is super expensive. Second, he's not that strong for his price. You really don't need to run 4, or again, possibly any.


Some zombies that you might have forgotten about:
Slitherhead
Spiteful Returned (this. guy. wins. games.)
Servant of Tymaret (good choice for your Obelisk of Urd)

Thought this looked fun.
I'm on the fence about how much mana to add. Having more mana lets me increase the size of the zombie train.More lands means fewer zombies. Hmmmm.