Also, hopefully, I'll keep to it. So, without further ado:
Kingmaker (the write-up assumes Lamentations of the Flame Princess rules)
The sword called Kingmaker appears as a khopesh of archaic manufacture with a gilded and jeweled hilt. One side of the blade bears the inscription "I am wielded;" the other side bears the inscription "I am sheathed." These inscriptions appear in an ancient language requiring a Languages roll at -3 to translate; Ancient Egyptian is a fair analogue for the culture from which the blade originates.
Kingmaker detects as magical if Detect Magic or similar abilities are used. It counts as a medium weapon dealing 1d8 damage. If found in a treasure hoard and sold by greedy adventurers, it can be sold for maybe somewhere in the 1,000-5,000 sp range. (No doubt someone might try to pry off the gems, scrape off the gold, and sell them separately; you could maybe get 1,000 sp or so for the components, and 10 sp for the sword — maybe 100 to 200 sp from a collector still willing to buy the well-crafted blade despite its obvious damage.) The merchant to whom you sell the blade will no doubt appear again, as suggested below.
The sword only has one effect: when the monarch of the closest kingdom to the sword's owner dies (Kingmaker's owner being defined as whatever living or unliving being is carrying the sword when the monarch dies; if the sword is stashed on a cart or something, then whomever most logically lays claim to that stash counts as the "owner"), Kingmaker manufactures evidence to place the owner next in the line of succession. It cannot manipulate memories or conflate truths and lies, but it can forge documents, shift personal effects, and even alter its owner's DNA on the off-chance someone has the magic or technology necessary to check. (This won't change the owner's characteristics at all, but will provide convincing genetic markers for anyone investigating the owner's lineage.) Despite the overwhelming evidence that the owner is some long-lost bastard child or cousin or whatever, the sudden appearance of this heir is always designed to cause the maximum possible amount of strife on a national scale. Affairs will be revealed, conspiracies will be unearthed, siblings will go to war, assassins will be unleashed, and the sword's unwitting owner will be at the center of all this, often with no real inkling as to why this is happening. Canny wielders may turn this situation to their advantage, of course, assuming they live that long.
The sword's owner remains the same, even if they divest themselves of the sword, until death. Of course, if someone kills the owner and takes the sword, then the sword manufactures evidence for that person, starting the cycle anew.
The sword is sapient and Chaotic in alignment, able to communicate telepathically with creatures within 100 feet, although rarely deigning to talk to anyone. The sword looks on living and unliving things with disdain, considering the works of man and other sapients as abominations against the natural order.
Incidentally, if the player characters stripped the sword of valuables and sold it, it will somehow communicate to its new owner that this is all the player characters' fault. (It may do likewise if they sold it unmolested, or maybe they'll just hear of the troubles when the merchant to whom they sold the weapon is named next in line to be king.)
It is entirely possible that the sword can manufacture evidence at will, sowing chaos among the cities of men.