I like the idea of changing AA to some type of sword magus, probably a kensai, and giving him Corpselight as an end-of-campaign reward/continued adventure hook, so here's my take on the lost blade. It takes inspiration from Raven's Head and the Corpselight Rapier from the Prince of Wolves chronicle sheet. I'm new to designing artifacts, so any advice would be welcome.
Corpselight
Aura strong evocation and conjuration; CL 20th Slot none; Weight 6 lbs.
DESCRIPTION
Corpselight was once an adamantine +2 holy undead bane bastard sword. Designed for hunting and dispatching the undead, the unsheathed blade glowed with holy light as if affected by a daylight spell when undead are within a 60-ft. radius. Unfortunately, its antipathy with undead also acted as a beacon, and undead within 1 mile of the blade can sense its presence and direction, even when the sword was sheathed. Since its capture by the Whispering Tyrant, however, it has been transformed into an adamantine +2 unholy humanoid (Human) bane bastard sword. The bearer of this evil weapon is affected by the Tyrant’s Whispers haunt (Pathfinder #48 page 19) and any living creature slain by Corpselight rises as a skeletal champion after 24 hours.
DESTRUCTION
If used by a lich to slay the sitting ruler of Ustalav, Corpselight’s blade rusts away to nothing, its power broken forever.
HISTORY
Once wielded by the crown of Ustalav as a symbol of power over darkness it was carried into battle against the armies of Tar-Baphon by the doomed Prince Ardurras Virholt II in 3204 AR. Its owner slain, Corpselight fell into the hands of the Whispering Tyrant but rather than destroy the blade the Tyrant took it for himself, its corruption a symbol to demoralize the living. Through foul magicks and alchemical transmutations the Lich-King twisted the once-noble blade from its purpose to a foul new one. Its magic was systematically unraveled and reforged into an obscene mockery of its former glory as Tar-Baphon infused it with a fragment of his own antipathy to life. Carried at the head of the Whispering Tyrant’s vanguard, it felled each of Ustalav’s would-be champions in turn, returning them to unlife and further bolstering the Tyrant’s army. Presumed lost during the siege on Gallowspire in 3827 AR, Corpselight has passed from hand to hand, slowly forgetting what it once was but ever seeking to return to Tar-Baphon.
RAMIFICATIONS
Few items are as steeped in reverence and revulsion as Corpselight, as it once served Ustalavic royalty, only to be turned against it by the very evil it was meant to destroy. Those who claim it should consider the following.
• Tormented by the Tyrant: While perverting Corpselight into a weapon of evil, the Whispering Tyrant succeeded in placing a small part of his soul within the adamantine blade, not enough to turn it into a fully-fledged phylactery but enough to create a powerful mental link between himself and the blade’s bearer. Anyone who carries Corpselight hears the Tyrant whispering within his mind in a cacophony of languages including Ancient Osiriani, Azlanti, Common, Infernal, and Varisian. These whispers eventually drive Corpselight’s bearer insane, urging them to do everything in their power to return it to the Whispering Tyrant. Doing so would require the breaking of the three seals powering the Great Seal imprisoning Tar-Baphon within the dungeons of Gallowspire and would mean unleashing one of the most powerful and dangerous necromancers onto Golarion once more.
• Icon of rulership: While a formidable weapon in its own right, Corpselight was also once carried by the kings of Ustalav. As such, it is the rightful property of Ustalav’s current monarch but turning it over carries with it the same problems detailed above. Therefore, to be safely wielded, Corpselight must be cleansed of its undead taint and restored to its former glory. Only casting an atonement spell upon the sword, followed by using Corpselight to destroy a lich’s phylactery, will restore it.