Ive personally tested it in Boros Energy, splashing black for Sephiroth and some Thoughtseizes, mainly because of how combo-heavy the current meta is
Thats not all though, they also read, Whenever Umbris or another Nightmare or Horror enters the battlefield under your control, target opponent exiles cards from the top of their library until they exile a land card. On average, youre only going to get around three cards per creature this way, but that still means that Umbris comes in as a 4/4 the first time around, assuming youve not exiled anything else, and then just continues to grow from there
Stats: 2/3 The best new legend from the Final Fantasy Holiday Release is also the best of the new cards overall
Spiked Corridor // Torture Pit gives you two solid effects that burn or slug decks enjoy
While none of these lands are individually expensive, a fully Rossed out mana base could get a bit pricey
The dream play would be doing that on Worldspine Wurm