Resolve the Cauldron Familiar ability, returning it from your graveyard to the battlefield.
Cauldron Familiar and Skeleton Crew trigger.
Resolve the Skeleton Crew trigger, creating a creature token.
Resolve the Cauldron Familiar trigger, causing each opponent to lose 1 life and you to gain 1 life.
Activate Shilgengar by sacrificing Cauldron Familiar.
When Cauldron Familiar dies, Ygra triggers, putting two +1/+1 counters on it.
Resolve the Shilgengar ability.
Repeat.
This wicked loop transforms your entire graveyard and battlefield into an engine of endless rebirth, powered by Ygra, Eater of All turning every creature into a Food artifact that can fuel Cauldron Familiar's return. As the Familiar repeatedly hops back and forth, Skeleton Crew takes notice of cards leaving your graveyard to flood the board with Skeleton Pirate tokens, while Ygra grows monstrously large with two +1/+1 counters every time a creature bites the dust. Shilgengar, Sire of Famine ties the whole morbid assembly together by letting you sacrifice that Cat to churn out Blood tokens, which can later mass-resurrect your fallen army if needed.