This clever loop relies on Darkest Hour to paint every creature on your board black, turning Teysa, Orzhov Scion into an endless engine that replaces every sacrifice with a fresh Spirit token. You begin by using Shilgengar, Sire of Famine to sacrifice a creature and produce a Blood token, which immediately triggers Teysa's second ability because the sacrificed creature was black. The resulting 1/1 Spirit is also black thanks to the global enchantment, perfectly priming the board to repeat the cycle.