When your opponent's build step starts, choose one of your units in up to X arenas. Each of the chosen units gets "Pay 2 Force Ambush 6" until end of turn.
Keywords
Ambush
Ambush X: A layered, activated ability that means, "If your opponent deploys, moves, or puts a unit into this arena, this unit does X dice of damage to that unit." Each Ambush can be used once per instance of a unit being deployed, moved, or put.