The character gains a flat 5% chance to inflict a critical hit on a target with every attack. A critical hit multiplies the damage dealt by the attack by 1.5 times (rounded down), applied before soak is taken into account. Doors, wards and stronghold fortifications cannot be critically hit.
For example, a Myrmidon hits a target for 7 points of Impact damage with a Haymaker. The target's defenses soak 4 points of Impact damage. The Myrmidon therefore deals 3 points of Impact damage to the target. If the attack were a critical hit the damage inflicted would equal
(base damage x 1.5 - soak = 7 x 1.5 - 4 = 10.5 - 4 = 10 - 4 = 6)
6 points of Impact damage.
Myrmidons gain this skill for free immediately upon selecting their class.