If a persist creature is used, it does not have a -1/-1 counter on it.
Otherwise, if an undying creature is used, it does not have a +1/+1 counter on it.
Steps
Activate
Shilgengar by sacrificing the persist or undying creature.
When the creature dies, its persist or undying ability triggers, returning it from your graveyard to the battlefield without a -1/-1 counter or a +1/+1 counter on it due to
Solemnity.
Resolve the
Shilgengar ability.
Repeat.
Results
Infinite LTB.
Infinite ETB.
Infinite sacrifice triggers.
Infinite death triggers.
Infinite Blood tokens.
Metadata
In 4879 decks according to EDHREC.
Bracket Tag
This combo has been estimated to be Exhibition (Bracket 1+).