Are we allowed to include non-D&D magic items? Because, if so, I have two from my Dungeon World game.
1.
The Divine Scale
Not, as you might think, a god-wrought item for weighing things, but rather a single scale shed from the...conflict/melding/interaction (it's aaaaancient legend, so details are obscure) of Bahamut and Tiamat. An incredibly powerful artifact, because it could become
anything, being literally a small chunk of pure creation, open, even
grasping for a concept to shape it into something real. We had a devil of a time safeguarding it and preventing its accidental premature use, but in the end my Paladin, Abraxxis, ensured that the temple that had enshrined it, which revealed the ancient and forgotten origins of Bahamut (and Tiamat), would be re-created in his 'adopted' home temple Orsulo (if not
the Temple of Bahamut, at least one of the biggest in the known world). An incredibly cool, flavorful magic item that my DM basically gave me
carte blanche to make, because the other two players had already had a special personal adventure that gave them a cool item/benefit.
2.
The Sword of Six Towers
Sorry Lowkey, this one'll piss you off: it's a Holy Avenger. But it's not like any Holy Avenger I've ever seen, and explaining why will require some (read: LOTS of) backstory.
[sblock]In our campaign world, called Io (named after the Father Of All Dragons, a "dead" god whose children/sundered halves are Bahamut and Tiamat), magic is a powerful and worldwide force. At some point in the distant past, Wizards learned how to tap into the currents of magical energy that run through the planet's crust, creating great and powerful Towers that give shape and focus to a particular school/philosophy/focus of magic, and empowering one representative of that Tower with incredible magical might. These Avatars and their underlings (collectively "the Conclave") mostly keep to themselves, preferring to study their magical arts and fight with each other over petty territorial and doctrinal disputes, but they come and go as they please in the lands of others and don't give a crap about the laws of other lands (many Towers, though not all, will simply abduct children with magical talent as soon as they're identified). The Conclave's five Towers--Black (Necromancy), White (Artifice), Red (Evocation), Gold (Enchantment), and Green (Conjuration)--were considered untouchable, and practically unreachable, even by the local Empire.
Then, our party happened. Heh. A power-crazed Kobold Wizard (not trained by the Conclave), through some fortunate story and lucky rolls,
blew up the Black Tower completely--down to its foundations--and, later, killed its unnamed and untitled Avatar, reducing his/her magic (and what remained in the tower's foundation) to a thumb-sized black gem. This also released a magical explosion of necromantic energy, not so much in the physical world as in the veins of magic below the surface, infecting the other Avatars with its evil and even temporarily tweaking the minds of run-of-the-mill Conclave wizards. As a result, the party was forced to defeat the Gold Avatar (styled "the Allmind"), who had gone mad from the malign influence and begun consuming the minds of everyone in the Gold's territory, planning to absorb everyone, everywhere. Abraxxis and the party's Fighter basically talked the Allmind to death, who "released" the power of the Gold Tower, turning it a dull grey...and producing a golden citrine gem. Due to a complex series of events, including efforts by both the White and Red Avatars to claim the drained tower, it became the new Rose Tower, full of the magic of story and song (a blend of the "passion" of Red magic, the "precision/recording" of White magic, and the "illusions/beguiling" of Gold magic), with the Novelist as its new Avatar.
Later, due to shenanigans involving impersonation of the Green Avatar by an alien (far too long a story to explain), my Paladin acquired an emerald-green gem as well (though fortunately Avatars of the Green typically left such a gem behind when they passed on, so the Tower was unaffected). He performed a significant service for the Incendiary (the Red Avatar), cleansing her of the necromantic taint that had twisted her mind (creating the "Inferno"); when she recovered from that experience, she gifted him with a fiery red gem. And, finally, for defending the Rose Tower against the Inferno and her army, the Novelist gifted him with a rosy quartz gem.
Which leaves only the White Tower. Unlike the others, where we largely came unbidden, the Archivist summoned
us, to attend to an issue on her behalf. When Abraxxis asked her if she could give him a similar boon to the one he had received elsewhere, she explained that she could not: this was not the first time that such a dangerous evil of magic had been unleashed on the world, and she held within her a beautiful, pearlescent gem, layered with both darkness and light, for it had sealed and contained the worst of the effects. To take it would be to unleash this, and all prior magics, that the many Archivists had contained throughout history--an unthinkable evil for my Paladin. However, she recognized that his sword was an artifact created by the White Tower--made long and long ago, just in case someone should gather together these solidified essences of magic. So instead of giving a gem, she ground down the metal pommel to add a pommel-stone, a smooth opal; not a true Avatar gem, but a re-dedication of the sword to its purpose.
This sword, incidentally, was recovered from a mercenary hideout, and we learned that it was taken from a murdered Knight of Bahamut in service at the temple in Orsulo. Long before any of the gem-stuff, I struck the fatal blow to a Vampire Ancient who had been plaguing Orsulo...but its corrosive evil proved too great for the blessings of the sword, and the vestiges of the vampire's spirit began whispering to my character, terrifying him enough to make him stop using it (even though it's magical). So our next adventure was to go cleanse the sword, in the headwaters of the river Arendel (or something like that, I could never remember how it was spelled), which springs forth from a mountain rumored to be inhabited by dwarves. Along the way, our (crazy) halfling Fighter thought he had to 'liberate' a cruelly imprisoned red dragon...by cutting it open and stuffing the last drop of the vampire ancient's blood into it, creating a terrifying abomination, an ancient vampire dragon! But our party's Thief, at my Paladin's request, cleansed the sword in the river's headwaters (having earned the respect of the blind dwarven monks who guarded it by fighting without killing), and literally swooped in to the rescue; pierced by the divinely-empowered blade, the vampire-dragon howled in pain and took to flight, blasting through the side of the mountain...with Abraxxis in tow! As it disintegrated into ash in the morning sunlight, he fell to the bottom of the glacially-cold river and Determinator'd his way out, exiting the water to the cheers of the orc tribes who had come to slay the "Doom" (dragon) imprisoned in the mountain; for slaying their hated foe, they named Abraxxis "Doomslayer" and pledged their loyalty to him should need arise, but that hasn't happened yet.
But enough with the digression on how I got it and how it became truly blessed. Originally, the sword had sockets for just two gems; however, when the DM got a
wonderful, awful idea to make the Black Avatar drop a gem, the sword magically reconfigured itself to accept
five gems instead--one for each tower of magic. Now, however, it will be a Sword of
Six Towers, which is a beautiful coincidence, as we had established six as a number sacred to Bahamut ("snake eyes" is called "the eyes of Tiamat" in this world; "boxcars" is called "Bahamut's wings"). The six towers being Gold, Black, Red, Rose, Green, and White, with the last providing the sword itself to house the other five's gems.
Although we have not specifically spelled out the sword's effects yet, we have two things to go on. First, the "normal" Holy Avenger effects, which are to grant the Paladin
all Paladin-specific moves (a serious boon in DW, where you only get a third to a half of your class's total options), boost the class's damage die from 1d10 to 1d12, and ignore the defenses of any "creature of Evil" (which, I presume, needs to be more than just enjoying the sound puppies make when you kick them). Second, the gems convey an additional feature based on the magic they're associated with, though only Black and Gold have been determined. Black, Necromancy "cleansed" by the sacred sword, grants the Cleric move Turn Undead. Gold, Enchantment, gives one chance per session to improve a social interaction, so a failed roll becomes a partial success, a partial success becomes a full, and a full success becomes fantastic (that is, beyond what could have been hoped for). For the remaining colors, I pitched a concept-idea (that I can't seem to find now...), but our game has gone on hiatus so we haven't nailed it down any further.[/sblock]
Or, in brief: More than half of our adventures have, intentionally or accidentally, involved the forging of this sword into one of the most powerful artifacts in the world. My character will use it at its full potential for at most a couple of sessions--before retiring (or dying, if the dice are against me). It is a mechanical representation of the completion of his personal journey, complementing the completion of his personal narrative, and a beautiful example of cooperative story development. It wouldn't be nearly as awesome out of context, as it straight-up just wouldn't generalize, but for me it's probably the coolest damned magic item I've ever encountered, let alone "possessed" (or, rather, will possess? It's complicated.)