Arcbound Reclaimer dies, its modular ability and persist ability trigger.
Resolve the modular trigger, putting
Arcbound Reclaimer's +1/+1 counters on another artifact creature.
Resolve the persist trigger, returning
Arcbound Reclaimer from your graveyard to the battlefield. It will not have a -1/-1 counter on it due to its modular ability.
Resolve the
Spawning Pit ability.
Repeat.
Activate
Arcbound Reclaimer by removing a +1/+1 counter from it, targeting an artifact card in your graveyard.
If needed, hold priority and repeat step 1 until
Arcbound Reclaimer dies as a state-based action due to having 0 toughness.
When
Arcbound Reclaimer dies, its persist ability triggers, returning it from your graveyard to the battlefield with two +1/+1 counters on it due to its modular ability.
Holding priority, repeat from step 1.
Results
Infinite +1/+1 counters on certain creatures.
Infinite artifact ETB.
Infinite artifact LTB.
Infinite artifact sacrifice triggers.
Infinite charge counters on a permanent.
Infinite creature ETB.
Infinite creature LTB.
Infinite creature sacrifice triggers.
Infinite death triggers.
Metadata
Bracket tag
OddballBracket 2-3+
Probably 2 or 3, but hard to classify
1
2
3
4
5
1 · Exhibition5 · cEDH
Bracket tags are estimated from the cards and results of each combo, as a guideline rather than a strict classification.