This infinite loop is orchestrated by Shilgengar, Sire of Famine, whose ability lets you sacrifice Matter Reshaper to generate a Blood token while also triggering Matter Reshaper's own death ability. As Matter Reshaper hits the graveyard, Mortuary steps in with its static ability to intercept it, placing the creature right back on top of your library instead of letting it rest there. From that exact top-deck position, Matter Reshaper's own triggered ability resolves by revealing itself and sliding straight back onto the battlefield, ready to be sacrificed to Shilgengar, Sire of Famine all over again. By cycling Matter Reshaper endlessly between the battlefield, graveyard, and library, you fuel an infinite engine that endlessly multiplies your Blood tokens.