Activate Shilgengar by sacrificing the creature that has Screams from Within attached.
When the creature dies, Screams from Within triggers, returning it from your graveyard to the battlefield attached to another creature you control.
Quintorius, History Chaser triggers, creating a creature token.
Resolve the Shilgengar ability.
Repeat.
This elegant loop leverages Shilgengar, Sire of Famine to repeatedly sacrifice a creature, turning each sacrifice into a Blood token while triggering a cascade of enters- and leaves-the-battlefield events. The real engine that makes this recursion self-sustaining is Screams from Within, whose mandatory death trigger immediately returns it from the graveyard to the battlefield attached to a fresh target whenever its host perishes. Meanwhile, Quintorius, History Chaser quietly watches the graveyard, triggering every single time Screams from Within departs to churn out an endless army of 3/2 Spirit tokens.