Cast Priest of Gix from your graveyard for its escape cost by paying 2 and exiling three other cards from your graveyard.
When Priest of Gix enters, it triggers, adding .
Activate Lord of the Forsaken's first ability by paying and sacrificing Priest of Gix.
When Priest of Gix dies, Vein Ripper triggers, causing you to gain 2 life and target opponent to lose 2 life.
Resolve the Lord of the Forsaken ability, causing you to mill three cards.
Activate Lord of the Forsaken's second ability by paying 1 life, adding C that can only be spent to cast spells from your graveyard.
Repeat.
This engine relies on Underworld Breach to let you repeatedly cast Priest of Gix directly from your graveyard for its escape cost, fueling those recursive casts with the three black mana the Priest generates upon entry. Lord of the Forsaken acts as both the outlet and the engine lubricant, allowing you to sacrifice the Priest to mill more cards while its second ability converts a tiny slice of your life total into the colorless mana needed to pay the activation cost. Finally, Vein Ripper watches over the slaughter from the battlefield, turning every sacrificial death into a lethal drain of life that closes out the game.