If Azir, Sovereign moves into a battlefield and his ability goes on the chain, but the opponent uses Fate of the Fallen (FoF) in response to move him back to base, does his ability fail because 'this battlefield' is now NULL?

Ruling: Yes, the ability whiffs. When Azir, Sovereign is moved back to base by Fate of the Fallen before his ability resolves, the ability fails to resolve because "this battlefield" no longer refers to a valid location.

Nuances:

• Fate of the Fallen must be played from arsenal (hidden) to be used as a reaction in response to Azir's ability
• FoF from hand is not a reaction and cannot be used in this timing window