Geralf's Messenger dies, triggering its undying ability, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
Geralf's Messenger enters the battlefield, causing an opponent to lose 2 life.
Activate Retribution of the Ancients by paying ({B} magic symbol) and removing a +1/+1 counter from Geralf's Messenger, giving any creature -1/-1 until end of turn.
Repeat.
Results
Infinite death triggers.
Infinite ETB.
Infinite lifeloss.
Infinite LTB.
Infinite sacrifice triggers.
Reduce any number of target creatures' toughness to zero.