Natural 20 roll, unless otherwise specified. Repeat the attack roll. If the second roll is a hit, roll damage as normal and add maximum damage. On a miss, maximum damage. If a second critical hit is rolled, roll again. On a hit, roll damage as normal and add 2x maximum damage.On a miss, 2x maximum damage. On a third critical hit, the target is instantly slain.
Critical misses function like critical hits, save for the opportunity for dire repercussions beyond the initial miss. A critical miss will not deal half damage.