Adarkar Valkyrie + Intruder Alarm + Quicksilver Gargantuan

Adarkar Valkyrie

Intruder Alarm

Quicksilver Gargantuan

How it works

1. Activate Adarkar Valkyrie by tapping it, targeting any legendary creature you control.
2. Cast Quicksilver Gargantuan by paying 5, 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 Quicksilver Gargantuan.
7. Resolve the Adarkar Valkyrie trigger from step 4, returning the legendary creature to the battlefield from your graveyard.
8. Put Quicksilver Gargantuan 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 Quicksilver Gargantuan to the battlefield from your graveyard as a copy of the legendary creature.
13. Repeat from step 3.

This clever loop turns the legend rule into an engine, using Quicksilver Gargantuan to continually copy a legendary creature and instantly sacrifice itself to state-based actions. Because Intruder Alarm untaps all creatures every time a new copy enters the battlefield, Adarkar Valkyrie stays freshly untapped and ready to protect whichever version is currently waiting in the graveyard. By alternating targets between the two entities, Adarkar Valkyrie resurrects them back and forth indefinitely, generating an endless cascade of entry and death triggers.