The Executioner is a unique monster introduced in UnNetHack. He is incredibly deadly, possessing a solid -2 AC and two extremely strong weapon attacks that can each hit for a maximum of 50 base damage, and can only be outsped by a very fast player.
The Executioner only appears in the lowest level of Sheol, and will always be generated with a cloak of magic resistance and the artifact battle-axe Cleaver; if Cleaver is already generated in that game, he will instead have a normal battle-axe.
The Executioner can wipe out even ascension-ready players in a couple of turns, and is assisted by various other monsters on the same floor, particularly blue slimes that can freeze you in ice. In addition to intrinsic stoning resistance preventing use of footrice corpses, his starting cloak and decent monster magic resistance nullifies the wand of death and dampens or outright nullifies most spell-based approaches; he has further resistances to cold, fire, and poison. Carelessly approaching him in melee without proper protection can spell instant death even if you have an amulet of life saving, due to his speed and multiple attacks per turn.
However, the Executioner's threat can be greatly reduced if you can liberate his weapon from his hands, especially if it is Cleaver; without the artifact's damage bonuses, his melee attacks are somewhat more manageable, but still very dangerous. The half physical damage attribute may save your life several times over compared to an actual amulet of life saving, especially if you can come into the fight unencumbered and wearing speed boots; this can give you valuable turns to retreat and/or use any means of healing you have available. You can also use a wand of teleportation against him to give you some breathing room; teleporting yourself will not work, as all of Sheol is a no-teleport level.
If you only want the hoard of items he guards (including a crystal pick and magic marker, both of which are guaranteed), you can run past him or teleport him away to get at the chests, then level teleport out (since levelporting ignores no-teleport levels); the chests furthest from the stairs will always hold the crystal pick and magic marker.
Despite resisting many ranged forms of assault, the Executioner can still be picked off at a distance due to lacking any range of its own; you can block his path using boulders from a scroll of earth or any traps that may appear in the room, and then unleash a storm of your best projectiles to bring him down. Spider webs and bear traps can also temporarily pin the Executioner in place. Though he has solid MR, the Executioner also has no sleep resistance, and is just as vulnerable to paralysis as every other monster; if you can survive a round or two in melee and have sleep resistance and/or a ring of free action, you can hit him with a potion of sleep or paralysis to immobilize him long enough for a highly-enchanted artifact weapon to kill him.
One player was able to successfully hit him with a spell of sleep and dispatch him with a well-enchanted Magicbane; this led to the suggested t-shirt message "I knocked off the Executioner and all I got was this lousy crystal pick".
Polymorphing into a black dragon allows you to dispatch the Executioner with ease, as he does not resist disintegration, and two blasts of breath is enough to destroy his cloak and then disintegrate him; note that this will destroy his weapon as well, so do not resort to this if you wish to obtain it unless you can get it off his person beforehand.
Executioners like to immediately execute adventurers who get trapped in their icy lairs. Challenging them in direct hand-to-hand combat would be headlessly stupid.
'Executioners execute!' [ Adeon ]