Activate Shilgengar by sacrificing the persist creature.
When the persist creature dies, its persist ability triggers, returning it from your graveyard to the battlefield without a -1/-1 counter on it due to Grumgully
Resolve the Shilgengar ability.
Repeat
This clever loop transforms graveyard recursion into a self-sustaining engine by letting Shilgengar, Sire of Famine convert your sacrifices directly into Blood tokens. Because Grumgully, the Generous constantly bolsters your non-Human creatures with an extra +1/+1 counter as they re-enter, he instantly neutralizes the self-weakening nature of persist mechanics. This allows you to continuously recycle creatures through the graveyard, generating endless sacrifice triggers, entries, and exits while fueling the Sire's ravenous hunger.