Wormfang Drake

If you don’t control any other creatures as the enters-the-battlefield ability resolves, you must sacrifice Wormfang Drake.

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.

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.