Assassination is an automatic skill that increases the chances of beheading on a backstab, circle, aim, or snipe. The assassination skill has a higher chance of causing beheads than the beheading skill on a per-attack basis, but the assassination skill is only checked on the aforementioned attacks, where as the beheading skill is checked on normal attacks. Assassination chance is increased further by the ambush stance. See showskill assassination for more details.

beheading assassination chances
Sometimes rather than hacking an opponent to death, an adventurer with a edged weapon and the beheading skill has a chance to cleanly separate their opponent's head from the body--quickly deciding the outcome of the battle. The factors for success are nebulous, but wielding a vorpal weapon greatly increases the chances of beheading. Of course, the biggest factor in beheadings is luck. Heads will roll! Beheading with regular weapon attacks can only occur with certain weapons, and this is only checked for once every 1024 hits for one-handed weapons and every 512 hits for two-handed weapons (beheading mastery can up to double these frequencies). These weapons and their behead chances are: Exotic: 0% Dagger/polearm: 1% Sword: 5% Axe: 25% These base chances can be increased as follows: Sharp/vorpal flags: +1%/+5% Beheading skill: +50% The shadowclaw stance sets the behead chance to 100%, both with or without weapons (but will not make a non-beheading weapon such as a flail, capable of beheading), while the razor claws skill grants a 50% chance while unarmed and fighting in the default stance. In addition to beheadings with regular attacks, certain attacks can cause beheadings as well, typically with much higher chance of success. The razor claws skill beheads once with every 256 successful rake attacks. Beheading through assassination is typically done with daggers and ranged weapons, or unarmed with help of the lethal hands skill (which also substitutes for the assassination skill in that case). While other weapons can be used for assassination as well, chances of success are halved. The chance for assassination to be checked varies from skill to skill, but in each case the ambush stance doubles it, and assassination mastery can up to double it again. When assassination is checked, the chance to assassinate is calculated as: base_chance = ranged ? (100 + assassination) / 2 : assassination extra_chance = 50 + anatomy / 4 (+2 if sharp, +10 if vorpal) chance = (base_chance * extra_chance / 100)% The frequencies of assassination checks for special attack is given below: Backstab: 1 in 32 Back Leap: 1 in 32 Aim Head: 1 in 64 Snipe: 1 in 64 Circle: 1 in 128 Paroxysm: 1 in 128 Rupture: 1 in 128 Slash Throat: 1 in 128