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.