A little ability to make some monsters all that much more deadly. Remember, in the KIWF system, all hit dice are rolled each combat and either stay on the table or are recorded somewhere.
Vicious: If this creature's attack causes someone to lose a hit die, roll another damage die and apply that against the next hit die (apply damage to armor first, as usual). If the monster is extra awesome, you may let it continue with the Unending Viciousness ability.
Unending Viciousness: As above, but it repeats again if another hit die was lost by the extra damage roll, and again until these extra rolls have repeated as many times as this creature's current HD.
Example: Grolluck the barbarianoid gets rammed by an umber worm with Unending Viciouness. It's attack is so brutal, it goes through three of his hit dice before he is mercifully knocked out of the way. If the worm had rolled high enough, it could have chewed of up to 7 of Grolluck's hit dice, so he should count his lucky stars, as well as his remaining hit dice.