The reason I’m thinking about this is “Ammit Eternal”. Don’t know if that’s the real name, as it was spoiled in Japanese. The translation, via MythicSpoilers, reads like this:
Creature – Zombie Crocodile Demon
Whenever an opponent casts a spell, put a -1/-1 counter on Ammit Eternal.
Whenever Ammit Eternal deals combat damage to a player, remove all -1/-1 counters from it.
That should work well with Hapatra, Vizier of Poisons
I’m adding the white to play one card, but of course it also comes with sideboard options. That one card being Grind // Dust.
Should the deck try to incorporate Archfiend of Ifnir? Not sure. It would be hard to do it, because many of the cycling cards just aren’t strong enough for constructed.
I’ve tried to setup the manabase in such a way that if you have to play a black source tapped on turn one, you’ll probably be able to play a green source untapped on turn two. That’s why all the Forests.