A domain deck is usually built as an aggro deck, and 2 mana to deal 5 damage to a creature or player is one of the best ratios in the game
Hex Parasite will do it, as will Vampire Hexmage, which also adds a first strike creature to your graveyard
A couple of sagas have been made non-traditionally, but this is definitely my favorite for how rich the colors came out and how much definition the various figures have despite being made out of the same material
Imperial Recruiter Power 2 or less might sound like a huge limitation, but Imperial Recruiter can fetch a huge swath of creatures from your library, from utility creatures to combo pieces
A changeling in the command zone makes it easy to build typal decks based on their color identity
9 Grim Haruspex Card advantage is essential in Commander, and Grim Haruspex is one of the best mono-black creatures to enable it