Infinite +1/+1 counters on a creatureInfinite death triggersInfinite creature ETBInfinite lifelossInfinite creature LTBInfinite mana creatures you control can produceInfinite creature sacrifice triggersInfinite untap of creatures
How it works
Activate Carrion Feeder by sacrificing Geralf's Messenger.
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, triggering itself and Intruder Alarm.
Resolve the Geralf's Messenger trigger, causing an opponent to lose 2 life.
Resolve the Intruder Alarm trigger, untapping all creatures.
Resolve the Carrion Feeder ability from step 1, putting a +1/+1 counter on it.
Activate Skeleton Ship by tapping it, putting a -1/-1 counter on Geralf's Messenger.