All of the Doctors in the set use the creature type Time Lord Doctor, with Time Lord being the first two-word creature type
This means you could spend the mana to activate an ability of an artifact you control or an artifact card in your hand or graveyard, for example
In the above example, if you see that there's not two lands on top of your library, that might convince you that you should probably mulligan that hand
While they may technically be colorless cards, some still have whats called a color identity when it comes to playing Commander
Since Mais ability causes life loss and not damage, this even works through The One Rings protection in older formats, so opponents cant use that to get around it
It demands careful life management, but if you build around it properly, Necropotence does what black does best: convert risk into control