You control a creature that has a +1/+1 counter on it.
Steps
Activate a creature using
Grist's first loyalty ability by putting a loyalty counter on it, causing you to create a 1/1 Insect creature token and mill a card, and then put a loyalty counter on the creature and repeat the process if the card was an Insect.
When the Insect enters,
Cathars' Crusade triggers, putting a +1/+1 counter on each creature you control. This causes the Insect to gain
Grist's abilities due to
Agatha's Soul Cauldron.
Repeat from step 1 using the Insect token instead.
Results
Infinite creature tokens.
Infinite creature ETB.
Infinite self-mill.
Infinite +1/+1 counters on most creatures you control.
Metadata
Bracket tag
SpicyBracket 3-4+
Probably 3 or 4, but hard to classify
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1 · Exhibition5 · cEDH
Bracket tags are estimated from the cards and results of each combo, as a guideline rather than a strict classification.