I'm honestly not particularly worried. For one, even if we assume that he's the end-all-be-all of broken cards, he's pretty bad outside of mono-black and possibly some Bx decks.
And in terms of being broken, I'm not particularly worried either. Functionally he feels to me very similar to cheater generals like original Kaalia and Jhoira, which while occasionally irritating and easy to build in an antisocial way are by no means broken. He's effectively a glorified ramp spell, providing no card advantage, removal, or board control in and of himself and being at best a B-rated win condition. He's also not one to leave much of a lasting impact if answered, and also has very little ways to protect himself.
Honestly, most of the problems I imagine him causing are ones that get pretty severely undercut by his dependency on a heavy black list. Counterspells, cheap draw, good removal that hits something other than creatures, these are all things that black does not excel at. Even looking at EDHREC, a website that tends to skew competitive, there wasn't anything I saw that particularly bothered me.
Pontiff of Blight,
Vilis, and
Blood Celebrant are the cards that seemed far and away the scariest, and of the three the Vilis interaction is the only one that I'd even argue is anything close to unique. The Pontiff of Blight combo is not far off from the billion other aristocrat-ish effects in mono-black, and Blood Celebrant effectively just turns into a color-shifted
Treasonous Ogre, and even Vilis is effectively just a better Null Profusion.
