Dreadhorde Invasion
Oracle Text
At the beginning of your upkeep, you lose 1 life and amass 1. (Put a +1/+1 counter on an Army you control. If you don't control one, create a 0/0 black Zombie Army creature token first.)
Whenever a Zombie creature token you control with power 6 or greater attacks, it gains lifelink until end of turn.
Seriously, though, I'm pretty down on the amass mechanic. While it has the potential to be scary if it appears incidentally on enough powerful cards (similar to energy), I imagine that it will fall closer to the outlast mechanic from Khans. While the thought of a "creatureless" grixis control deck holds some appeal, I keep flashing back to the awaken mechanic from BFZ. No matter how big and scary you build up that vanilla, it only takes one kill spell to knock you to square 1.
We'll see what sort of support it gets.
I think this might not be nearly as useful to the deck as Bolas' Citadel, considering I run Song of Freyalise to make all those Zombies into mana dorks! But we'll see if there are any good Zombie Army creature cards, or if they'll only ever be created through spells with the amass keyword.