You control any number of creatures that don't have summoning sickness.
Steps
Activate a creature you control by tapping it, causing a player to mill cards equal to that creature's toughness.
If a creature card was milled this way,
Zellix triggers, creating a 1/1 Horror creature token.
If a Horror entered,
Intruder Alarm triggers, untapping all creatures.
Repeat.
Notes
The combo can fail if you tap each of your creatures and fail to mill a creature card. It is recommended to have as many creatures without summoning sickness as possible to reduce this chance.
Results
Near-infinite mill.
Near-infinite self-mill.
Near-infinite creature tokens.
Near-infinite untap of all creatures.
Near-infinite creature ETB.
Metadata
Bracket tag
SpicyBracket 3-4+
Probably 3 or 4, 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.