Displacer Kitten and Eternal Witness on the battlefield.
You have an instant or sorcery card in your graveyard that has legal targets and that can be cast using ({B} magic symbol) ({B} magic symbol) ({B} magic symbol) .
You have at least eight or more cards in your graveyard.
When Eternal Witness enters the battlefield, it triggers, returning Cabal Ritual from your graveyard to your hand.
Resolve the instant or sorcery.
Repeat.
Results
Infinite black mana.
Infinite casts of any instants and/or sorceries in your graveyard that can be cast using ({B} magic symbol) ({B} magic symbol) ({B} magic symbol) ({B} magic symbol) .