Each Umbaran gets +10 speed and "Pay 1 Force Ambush 1 (If your opponent deploys, moves or puts a unit into this arena, this unit does 1 dice of damage to that unit.)
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.