Backfire X: A cumulative ability triggered by the initial dice roll that means, "When you roll at least one natural three with this unit's attack dice before re-roll, this unit does X dice of damage to itself when the attack ends." Backfire applies only to dice rolled for an attack, not to dice rolls for abilities like Retaliate.
Surge
Surge X: A cumulative, triggered ability that means, "Whenever another unit damages this unit, you may give a unit of your choice in this arena +X power until end of turn."