Whenever Miara or another Elf you control dies, you may pay and 1 life. If you do, draw a card.
Partner (You can have two commanders if both have partner.)
Miara turns dead Elves into card advantage, so you flood the board with cheap Elves and aristocrats fodder, then sacrifice them for value while draining the table. As a Partner commander with a low mana value, she's most often paired with another partner (frequently a green or other-color one) to push into a tribal Elf or aristocrats engine. Win through repeated drain, an overwhelming Elf swarm, or a sacrifice combo.
Cheap, two-mana commander that converts your dying creatures into a steady stream of cards
Partner lets you splash a second color or a second engine commander for far more flexibility
Slots perfectly into Elfball/aristocrats shells where creatures die constantly anyway
Card advantage in mono-black, a color that traditionally pays life for cards, fits naturally
Each draw costs and 1 life, so the engine taxes your mana and life total under pressure
Pure mono-black Elf support is thin; you really want a partner or green to enable Elf tribal
Board wipes both feed and gut her—you draw, but lose your whole engine
Does nothing on her own without a sacrifice outlet or a wide, expendable board
Pumps the team and generates explosive green mana to fuel Miara's repeated activations.
Gravepact
Edicts the table each time an Elf dies, turning your sacrifice engine into a board-control machine.
Add a partner that complements your plan—an Elf or green ramp partner for tribal, or a value/aristocrats partner for sacrifice synergy. Tighten the manabase and add free sacrifice outlets (Ashnod's Altar, Phyrexian Altar, Viscera Seer) plus aristocrats payoffs (Blood Artist, Zulaport Cutthroat) so each death does triple duty. Finally, include recursion like Eternal Witness effects and a tutor package to assemble combos consistently and protect against board wipes.
Legendary Creature — Elf Scout
and 1 life. If you do, draw a card.