"The man in black fled across the desert and the gunslinger followed."- Stephen King, The Gunslinger
A gaunt revenant stalks the wastes of the world of Arn.
No one knows what sorcerer summoned this entity from some alternate material plane, nor what sorts of strange magics animate his deathless form, nor what blasphemies are held back by his rawhide-sown lips. Perhaps he was called forth with the sepulchral sorceries of the Tomb-Lords of Amenti, or perhaps he's wandered since the days of the God Makers, themselves.
Whatever the case, the grim walker may be encountered randomly in sparsely inhabited deserts and badlands across the world. He is a tall, almost skeletally thin man. His weathered skin has the corpse-pallor of the undead. Beneath the wide brim of his hat, his eyes glow with blue-white witch-fire. His dust-colored long-coat flaps in the breeze--even when there isn't one. 'Round his waist is a low-slung leather belt, and odd scabbards--holding even odder weapons--hang on either side, strapped to his thighs.
The sole weapons of the man are cunningly wrought gonnes in miniature. These weapons fire fast, and they never need to be reloaded. Those who don't die on the battlefield from their eldritch shot typically succumb later to a wasting sickness, unless clerical magic is used to cure the disease.
This bounty-killer from the Outer Dark may be hired by leaving a piece of paper, with the likeness of the person to be killed drawn upon it, in a ruined (and so unconsecrated) church of a lawful good deity in a ghost-town in the wilderness, or at the edge of a desert or waste. He will only accept a commission from a person once in their life. The price for the killing is variable, but always includes gold--and ages the would-be employer in the bargain.
Mechanics: The Gonne-Slinger can be tailored to fit the needs of the campaign. Warriors & Warlocks stats for my current game aren't as easy, but here's some Old School-ish guidelines: I'd suggest a rough level of say the AD&D Fiend Folio Death Knight (AC 0, HD 9, Magic Resistance: 75%) though with a number of attacks reflecting the RoF of his guns (3 shots/round, maybe?). The cost of the killing should be based on the hit dice of the target, and perhaps the aging should, as well. Despite his undeadish nature (and the resistances/immunities that might imply) the Gonne-Slinger can't be turned, commanded, or destroyed, and is immune to the effects of holy/unholy symbols, holy water or the like.