Requires

Adarkar Valkyrie must be on the battlefield. Intruder Alarm must be on the battlefield. Undercover Operative must be in hand. Adarkar Valkyrie does not have summoning sickness. You control at least one legendary creature. Mana needed:

Steps

1. Activate Adarkar Valkyrie by tapping it, targeting any legendary creature you control.
2. Cast Undercover Operative by paying , 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 Undercover Operative.
7. Resolve the Adarkar Valkyrie trigger from step 4, returning the legendary creature to the battlefield from your graveyard.
8. Put Undercover Operative 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 Undercover Operative to the battlefield from your graveyard as a copy of the legendary creature.
13. Repeat from step 3.

Results