Once he has died once, you need to hit seven mana, in a deck without green, to replay him
MTG has many mechanics that revolve around artifact numbers and density, so we have not only affinity cards here, but also improvise, metalcraft, and other artifact-matters mechanics and cards
In any case, the black Phyrexian Dominus is (unsurprisingly) a great card for aristocrats
The second ability gives very relevant protection, and its a wonder with mana dorks as it gives you an outlet for their mana and repays your Llanowar Elves by making it into a real creature
If you are lucky enough to find this card in a booster pack, you are looking at some significant value
Its a fantastic way to protect your party members and continue to reap the benefits of having a full party