Adarkar Valkyrie + Intruder Alarm + Protean Raider

Adarkar Valkyrie

Intruder Alarm

Protean Raider

How it works

1. Activate Adarkar Valkyrie by tapping it, targeting any legendary creature you control.
2. Cast Protean Raider by paying 1, choosing to have it enter the battlefield as a copy of the legendary creature targeted in step 1.
3. Put the legendary creature into your graveyard due to the legend rule.
4. Adarkar Valkyrie and Intruder Alarm trigger.
5. Resolve the Intruder Alarm trigger, untapping all creatures.
6. Holding priority, activate Adarkar Valkyrie by tapping it, targeting Protean Raider.
7. Resolve the Adarkar Valkyrie trigger from step 4, returning the legendary creature to the battlefield from your graveyard.
8. Put Protean Raider into your graveyard due to the legend rule.
9. Adarkar Valkyrie and Intruder Alarm trigger.
10. Resolve the Intruder Alarm trigger, untapping all creatures.
11. Holding priority, activate Adarkar Valkyrie by tapping it, targeting the legendary creature.
12. Resolve the Adarkar Valkyrie trigger from step 9, returning Protean Raider to the battlefield from your graveyard as a copy of the legendary creature.
13. Repeat from step 3.

This clever loop harnesses the constant state-based removal of the legend rule to repeatedly churn a legendary creature and Protean Raider in and out of the graveyard. Because Intruder Alarm untaps all creatures whenever anything enters the battlefield, Adarkar Valkyrie is constantly refreshed to tap again and target whichever piece just fell to the graveyard.