When The Master dies, if it was a copy of the undying creature, its undying ability triggers, returning it from your graveyard to the battlefield as a copy of the persist creature with a +1/+1 counter on it.
Otherwise, if The Master was a copy of the persist creature, its persist ability triggers, returning it from your graveyard to the battlefield as a copy of the undying creature with a -1/-1 counter on it.
Resolve the Shilgengar ability
Repeat.
This ingenious engine turns The Master, Formed Anew into an immortal shapeshifter by exploiting the interplay between undying and persist mechanics. When you feed The Master to Shilgengar, Sire of Famine, its death triggers the ability of whatever creature it was mimicking—supplying the precise counter it needs to bounce right back as the *other* creature from exile. Meanwhile, Shilgengar happily converts those sacrificed bodies into Blood tokens, which can eventually be cashed in to flood the board with your entire graveyard.