While 6 mana is a lot, he is a removal spell and a lord at once. Overseer of the Damned is often a good card in my playgroups, so I definitely see this being played in every Vampire deck.
The Bolas deck is a weird mix of aggro with Afflict, and control with a lot of removal and Bolas itself.
Nissa's deck is a mass of ramp and drawing, trying to win with either Nissa, the Behemoths, or card advantage.
The rares for both decks are good choices IMO, I may get the Bolas one.
The ability is definitely powerful but narrow.
Halimar Depths and Skyline Cascade are useful too if you want more lands with effects. There's also Blighted Cataract, but that's more of an Eldrazi flavor card.
The Bolas deck is a weird mix of aggro with Afflict, and control with a lot of removal and Bolas itself.
Nissa's deck is a mass of ramp and drawing, trying to win with either Nissa, the Behemoths, or card advantage.
The rares for both decks are good choices IMO, I may get the Bolas one.
I hope the other 2 legends get more interesting abilities.