The Thorncrown of Iomedae—or, as it is known to the faithful of Iomedae, the Truecrown of Iomedae—is a major artifact made from the blasted remnants of Iomedae's helmet left after she succeeded in the Test of the Starstone.1
The Thorncrown appears to be a wooden circlet impaled with jagged spikes of metal, the remains of Iomedae's helmet. When worn by a paladin, it transforms into its true form: a brilliant halo of divine power.1
For any wearer, the Thorncrown of Iomedae functions similarly to a crown of blasting. However, the Thorncrown's true powers are unlocked only when worn by a pious paladin: it enhances their healing and merciful touch as well as their ability to smite evil. If a paladin is slain while wearing it, the Thorncrown of Iomedae can even briefly resurrect the paladin to continue their sworn duties before passing on.1
The Thorncrown harms any evil creature who wears it, and any paladin who breaks their code while wearing the artifact is instantly disintegrated.1
As Iomedae stepped forth from the Starstone Cathedral, her armour melted from her new divine form. She soon departed to serve her god Aroden, and her new followers gathered the molten remains of her armour to be forged into holy relics of her new religion. Her helmet survived more intact than the rest of her armour and has become known amongst her followers of the Truecrown of Iomedae. The Truecrown was returned from Absalom to distant Lastwall by the knightly order Iomedae had once led, the Knights of Ozem. Upon reaching Lastwall's capital of Vigil, her dedicated knights began work on Iomedae's first cathedral, the Sancta Iomedaea, to house the Thorncrown.1
Theft and resurfacing
However, on the eve of the cathedral's consecration, the Thorncrown's guardians were found slain and the artifact was gone. No one knows who stole the Thorncrown of Iomedae or where it went, but 15 years later it emerged in the possession of the Pathfinder Society. The burgeoning Church of Iomedae petitioned the Pathfinder Society to return the sacred artifact, but were ultimately ignored. The Thorncrown then moved between various Pathfinder lodges for 800 years as various experts tried to unlock its mysteries, including any kind of insight into the Test of the Starstone.
Disappearance from Galt
The Thorncrown was located in the Pathfinder lodge in Woodsedge when the Red Revolution swept Galt, and it was lost to looters.12 Since then, reports of the Thorncrown have surfaced in places as distant as Qadira and Varisia, where both the Pathfinder Society and Church of Iomedae still hunt for the missing relic.1
One purported spotting of the Thorncrown occurred in the Fangwood within Lastwall, by the Iomedaean paladin Benner Lightbourne during his infiltration of a criminal band of Nirmathi smugglers. He learned of a magical crown resembling the Thorncrown being stolen from an agent of the Aspis Consortium and fenced to an unknown woman en route to Tamran. However, he was unable to retain aid from the Knights of Ozem on rumor alone, and was captured by the apostate devil Vipostix while attempting to locate it on his own.3
For additional as-yet unincorporated sources about this subject, see the Meta page.