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Flying
When Wormfang Drake enters the battlefield, sacrifice it unless you exile a creature you control other than Wormfang Drake.
When Wormfang Drake leaves the battlefield, return the exiled card to the battlefield under its owner's control.
If Wormfang Drake leaves the battlefield before its enters-the-battlefield ability resolves, the “leaves-the-battlefield” ability will trigger and resolve first. It won’t return anything. Then the enters-the-battlefield ability will resolve. If you exile a creature you control, it will be exiled indefinitely. Wormfang Drake won’t return it.
If you don’t control any other creatures as the enters-the-battlefield ability resolves, you must sacrifice Wormfang Drake.
You choose which creature you control to exile (if any) as the enters-the-battlefield ability resolves. Once you make that choice, no player may take actions until the chosen creature is exiled or Wormfang Drake is sacrificed and the ability has finished resolving.
