Quickness (This Unit can attack or activate Exhaust abilities the turn it is played.) Arrive (When this card enters the field zone.) – You can sacrifice a Gear you control to destroy target Unit your opponent controls.
Q: Does Corum Assassin’s Arrive ability need to sacrifice a Gear as a cost?
A: No. When the Arrive ability goes on the Queue, you select which target unit you are attempting to destroy. When the ability resolves, you may sacrifice a Gear, and the unit is destroyed. If you don’t control a Gear, or if you choose not to sacrifice one, then the unit is not destroyed. (See costs of triggered abilities – 18.104.22.168)