A little backstory: When I was in college, one of the only available AD&D games was run by a guy who always had an NPC accompany the party. This NPC was invariably a character the GM played in a game 'at home', and was both higher level than the PCs, had managed to 'naturally' roll 18's in every stat and max hit points, and generally had some selection of either the best loot from the back of the DMG, some unique items from a module, or some homebrew monstrosity of a magic weapon, or a mix thereof. Generally, the adventure would revolve around the NPC leading the party into some fantastically dangerous high-level module that the NPC wasn't quite strong enough to solo, so that the NPC could acquire some specific piece of gear in a semi-legitimate fashion so he could then return to his 'home' campaign at an even higher level of power. You guys probably know the type. Thus:

170. The Slumming Demigod. This 'hireling' is a level 1.5X (where X is the average level of the party) half-dragon/half-god (from the optional rules published in Dragon Magazine Issue XXX!) dual-classed Dimensional High-Lord (from the optional supplement published by -----------, which allows dual-wielding greatswords while casting Magic-User spells at the same level, with Barbarian hit dice and Paladin saves)/Psi-Assassin (from... oh, who gives a flip?). The Demigod only shows up and offers his services when the party is either known to be heading for a place of interest, or if he can steer the party in the desired direction. This tall, muscular, handsome, witty gentleman, of urbane intelligence, tends to avoid the limelight, but when bored will challenge party members to drinking or arm-wrestling competitions, which he invariable wins. In a fight, he generally does not take the lead, but will wander around the periphery of the action, exterminating minions in a single blow... without ever missing, not even once. He only intervenes when it is absolutely necessary, if the PCs are on the ropes against the big boss, at which point the Demigod, having taken only cosmetic damage at best due to an insanely high AC, a Ring of Regeneration, and having made all his saves, will proceed to wipe the floor with an opponent who nearly TPK'd everyone else. At this point, he will claim the most unique and powerful item of treasure as his just reward, and vanish into the sunset.