Lord of the Forsaken

The third line is where this Demon earns its keep, and it is easy to skim past. Paying life for colorless mana usable only to cast a spell from your graveyard turns the creature into a private ramp engine for recursion strategies: flashback, disturb, escape, anything that recasts a spell out of the yard. Note the exact word: it pays only to cast, so activated abilities like unearth or the various "activate from your graveyard" costs are out; this fuels spells, not activations. Most rituals and mana rocks feed your whole hand indiscriminately; this restricts the mana to a single job, which is precisely why it enables loops that generic acceleration cannot. The other ability, paying and sacrificing another creature to mill three, is not a bolt-on rate-filler: it actively stocks the graveyard the mana ability wants to draw from. Together the two describe one closed system: convert bodies (plus black mana) into fuel, convert life into the means to recast it. A 6/6 flier with trample is a fine top-end on its own, but that stat line is the least interesting thing here; it is the reason the card survives long enough to run the engine twice. The friction is your life total and the recurring -plus-a-body sacrifice cost, both of which scale badly if you lean on the mill every turn, and that is the balancing act keeping a repeatable graveyard-mana faucet from spiraling. Read it as a payoff for a graveyard deck already built, not the card that builds that deck by itself.