The Qij-na the Shattered theory is cool--nice work, ripvanwormer! But here's my OD&D and weird fiction-inspired take...
Legacy Leech theory
by Patrick SullivanSince before the earliest memories of today's immortals, there have been a few mortals in each generation gifted with extraordinary magical ability but cursed with madness, recklessness, and cruelty. And a few of these mad wizards gain such knowledge and power that they begin summoning horrors from the abyss. Turning one of these fiends loose to wreak havoc is bad enough, but again and again over history, there have arisen dark sorcerors who go further, seeking to bind these extraplanar demons, trapping them in a vessel and forcing them to serve. There is something about swords that make them the ideal vessels for such intellects, but other weapons and even nonweapon objects have been used from time to time. The dark rituals for summoning and binding are buried in the most vile tomes, but some common elements are known. Summoning such fiends requires the blood of a dragon; the murder of an innocent; and the anguish of a multitude. Binding them to the Prime Plane and into a vessel requires earth, air, fire, water, and steel.
And for just as long, spellcasters dedicated to the defense of Mystara and its people have sought to remove such threats when identified: actually destroying the fiends is beyond the capability of most mortals, even when working in concert, so other approaches are necessary. The high priests and druids of many faiths guard secret and complex rituals that they employ only when they locate such a vessel of evil. Through complex ceremonies and prayer, they can expel the fiend's essence from the enchanted vessel, and then banish it across the fabric of reality, into the dimension of nightmares. And thus the never-ending battle of good and evil continues.
And that would be that... except that, on the other side of the infinitely-thin veil between worlds, a similar battle of good and evil rages endlessly through the ages. Dark sorcerors in the dimension of nightmares similarly bind extraplanar fiends in steel, and Diaboli and other priests and druids perform secret rituals to sever the connection between demon and weapon, and then banish the fiend into a neighboring dimension. The dimension that happens to include Mystara.
Disembodied, the spirits of these fiends rage voicelessly against their long imprisonment within a mere object and the world which they can no longer experience. Vicious, angry thoughts, lacking even the manifestation of a ghost, drifting aimlessly through an alien world they can barely sense. Just as the mortals within the Prime Plane and the Dimension of Nightmares can barely sense them: a passing bad mood; an unexpected flash of rage; vague echoes of inchoate gibbering; disturbingly-writhing tentacles glimpsed out of the corner of the mind's eye.
Over centuries or millennia, most of these disembodied fiends's essences have drifted beyond the skyshield and out into the uncharted void. But a rare few have found a different fate in recent centuries. They have managed to drift into a magical material that combines the core ingredients of the enchantments that work on their kind: blood of the Great One which rained down through the air; ashes of murdered enduks; Ixion's fiery wrath and spellcraft; anguish from many races and peoples; earth and water melded into weird red clay; steel seed and cinnabryl. The vermeil-infused red clay would catch a spirit, serving as a substrate as it re-incarnated and began slowly building a new body. Each of these fiends began its new life in the larval stage common to all fiends, a microscopic sac-like tube with hair-thin tentacles at either end. Sheltered within the red clay, they slowly grew in the coming years, bloating into the slug-like forms associated with legacy leeches. Despite their small, larval bodies still in early stages of development, the legacy leeches still carried the same rage and frustration born through their centuries of exile: angry turgid sacs of filth, feebly writhing in the sunless clay.
And it was this silent rage that attracted Her attention. You see, She had been there when the red clay nursery was first laid down. Nervous about Thanatos' rising ascendancy in the world and the Nithians' persecution of Her followers, she allied herself with Ixion, the Great One, and their allies to form the ad-hoc Council of Oblivion, partnering with Her fellow hierarchs to take direct action against the Nithians, Manscorpions. and Herathians. It was She who, under cloak of darkness, descended beneath Surra-Man-Raa and hid the Pearl. It was Her sorceries that turned entropy upon itself. It was Her secret incantations that bound together the magicks of Her fellow councilmembers: blood of the dragon, anguish of multitudes, and the rest. The rest of the Council seemed uninterested in the lingering effects of their collaborative efforts far to the west. But She sensed potential in the odd fallout, and She watched and waited. She watched, too, for attention from Her former partners--never quite allies--but they stayed away. And it was during this long observation that She first noticed the rage from beneath the surface. Taking incorporeal form, She descended to investigate and found something that reminded Her of the larval fiends in the creches back home. And yet the rage she felt from this little fiend far exceeded anything She had ever previously felt from larva. Fascinated, She returned again and again to observe, gurgling traditional lullabies to the young tentatcle-monster and, over time, discovering more and more of them.
Three centuries ago, She watched as the eldest burrowed its way to the surface. She was intrigued to see that it was more red-colored than usual for young fiends, and that it seemed to glow faintly, disturbing the darkness of the night, much like the vermeil dust that blows off the red clay deposits when the weather is dry. When the sun rose the next morning, She was still observing, so She saw how it instantly began drying the young fiend's unctuous flesh. And She saw how it instinctively reverted to a new form: a rapier, shining red in the sun. She saw the party of dwarves approach, though they did not see Her. And She observed as one of the dwarves bent to pick up the strange sword, and as the young legacy leach pierced his hand and fused. The other dwarves treated this development with suspicion, so she put them to sleep, and then watched in fascination as the red steel rapier wielded by the lucky dwarf pierced the heart of each of his former companions in turn, its might swelling as it drank the powers from each. Throughout the day and night, She stayed and conversed with the dwarf, learning of the fiend's history. Seized from its homeplane by strong magicks; bound to slavery within the vessel of a rapier by some horned sorcerer, residing so long within the rapier that it felt like home. And then forcibly expelled from the rapier by diabolus druids, driven into another dimension, and cursed to wander in silence and disincarnate rage through this alien world. When the sun threatened, reverting to the rapier form felt natural; when the dwarves were incapacitated, drinking their hearts' blood felt even more natural.
When She went next to visit the other larvae, She began gurgling to them of how to transform into a rapier. Over time, She has watched each and every one, learning about additional weapon vessels and passing along instructions for how to disguise themselves. She has tunneled ley lines through the vermeil veins, allowing the young demons to communicate directly with one another, rather than rely on Her as intermediary. And when the eldest had drained its twentieth legacy and began its abrupt metamorphosis, She was alongside to make sure that it found sufficient food and then to evacuate it to her Home plane, where it could thrive. She alerted some of the others of the important potential of drinking legacies, and allowed them permission to share the tips with one another, too. The red clay continues to trap and foster more shades, and more and more of these demons are growing sufficiently experienced to strike out on their own.