Whenever one or more nontoken Vampires you control attack, create a 1/1 white Vampire creature token with lifelink.
Mavren Fein turns every combat into a token engine: cast nontoken Vampires, swing, and flood the board with lifelinking 1/1s. You grow wide, gain life, and convert the army into a lethal alpha strike with anthems and overrun effects. Early turns develop ramp and cheap Vampires; mid-game you snowball tokens and protect the team.
Free, repeatable token generation just for attacking with creatures you already want to play
Lifelink tokens stabilize you against aggro and keep you out of burn range
Mono-white means consistent mana and access to the best go-wide anthems and protection
Plays well with extra-combat and double-strike effects that multiply triggers
Heavily reliant on combat, so Fog effects and pillowfort decks stall you
Vulnerable to board wipes that erase your whole token army at once
Mono-white lacks card draw and ramp, so you can run out of gas
Tokens are tiny and easy to chump or sweep without anthem support
Legendary Creature — Vampire Cleric
