The Wailing Wall is a mountain range both tall and inhospitable; fervent volcanoes spew forth molten rock through glacial crusts, storms drive heavy clouds against them to bleed them of driving chilling rain, and ferocious mistakes of creation come here to hide.
It takes a peculiar kind of folk to settle in such a locale; as ever, a peculiar kind of folk will settle it though.
Driven out for heresy, the 7th Legion of the Cessian empire carried their pagan idols on palanquins onward, far from civilized lands, until the end of the world - the Wailing Wall.
Ruins of humble townships in a score of styles told the grim tale of those who had come before to this exile of the dispossessed.
Followers of dead gods, loyalists of fallen realms and obscure cults had perished in the chill heights, without sating the mountains' hunger.
The Legion might have perished too, if not for the Fleshless. So grave was the crime of that lost tribe that gods forbade to speak of it, and even the sinners themselves long had forgotten, lingering away without a limb to their name, gathering misshapen bodies of the remnants of previous outcasts by sheer force of will.
As winters mounted and supplies ran thin, the resolve of the Legion faltered. It was in their hour of need that the Fleshless came, offering their aid and shelter; in turn, they would receive a chance to live again.
In the Longest Night, when even gods all sleep, the Lost Legion chose half their number, the dim and the meek. With curved knives made of fangs, they were cut open, and the Fleshless allowed to creep within, and hide within flesh from the wrath of the gods.
The sun come once more, none of the cursed tribe were to be found, and if the divine flock ever noticed, they thought 'good riddance'.
So thorough had been the curse of the Fleshless that even their souls had lost all capacity to maintain a body; their new bodies molten and infirm. "Where is your half of the bargain?" the priest-general inquired, ready to command his troops to attack.
Filled with despair and treachery, the Fleshless struck without warning, seeking to engulf the humans with their blubberous forms. Yet, with a human in their amorphous coils, their souls gleaned how to give a body shape, briefly at last.
The battle halted as the struggling pairs stopped battle, their minds connected, the understanding sprouting.
"We shall join, and live - or part and die" they knew, and the whispers carried throughout the mountain valley.
"Join and live. Part, and die..."
Time has passed, and the two people are long one - yet each time children are born, their history manifests. Inevitably, two will be born, of the same gender, one human, one a writhing flowing thing. Alas, they join, the Guide supporting the Guardian with his firm body, and the Guardian shaping his body into garb, armor and armament. With time, the Guardian learns to create sturdy keratin or adapt rock slabs into his hide; especially talented ones learn to cover themselves with volcanic fire and mountain ice, or grow sharp and resilient crystals. The Fleshless had lived in the Wailing Wall for millennia, and learned to withstand the fury of the elements; the Guide is safe within.
The proximity to their cursed halves is evident in the Guides as well - numerous are the children that are born with strange additional appendages, extra eyes, strange organs or weird genders. Yet, especially the fittest survive.
The Guide carries his sibling, and his soul allows the Guardian to keep a form; while the Guardian may for example form a crystal blade, it is the strength of the Guide that drives it.
Within the Guardians, though, the hidden memory of their kind lingers, stowed deep within the dark recesses of their flawed souls. To listen to the ancestral remembrance may spell enlightenment - or madness. Therein lie distorted images of ages past, names and rites deities best left forgotten, and secrets that accuse reality of falsehood. The greatest secret of all is the suppressed memory of the Sin, buried in the midst of a labyrinth of increasingly maddening remembrance, twisting all thought that comes near.
The Life of the Joined
The organisms of the Guide and the Guardian become interconnected, their blood one; most often, what is sufficient to slay one spells doom for both.
Rarely does only one perish - and the fate of the Disjoined is very uncertain. If the Guide lives, he will be very much a handicapped cripple in the eyes of his kin, and much less suited to survive their harsh homeland. If the Guardian survives, his life is most often short, for as the soul of his Guide passes on, he becomes shapeless and meek. Only if he manages to find a fitting host will he live.
Rarely does it happen that a disjoined Guardian finds an outsider soon enough, one willing to accept that slithering thing to be his second skin; still, a few Guardians have ventured off into the world with their new Guides. There, they find that the 'curse' breeds true - a female Guide will find that if she becomes pregnant, hers will be twins, one of them a Guardian; a male Guide will secrete chemicals to induce multiple ovulation in his partner, increasing the likelihood for twins as well.
The minds of the Joined are in concert - while the voice of the Guide is heard, he rarely speaks without his twin's counsel. There is little barrier in this intimate telepathy - the two are separate, but one. To an adult (an outsder, perchance) newly joined, this is akin to possession by a spirit strange, when thoughts that are not his own begin to be stored along with his memories - and so it seems to those not of the Wailing Wall, when the Joined speak of themselves as two, and discuss matters with half of the conversation inaudible.
The Workings of the Shroud
Guardians appear very obviously organic - they writhe, pulse, open eyes in strange places, and may even extend half-formed appendages.
By honing their transformative skills and willpower, Guardians master advanced forms:
*Soon, every Guardian can transform himself in basic ways - grow fur, scales, bony armor plates, or claws and spikes. These powers can become more potent by absorbing the remnants of an extinct beast or a tribe that is no more.
Thoroughly ruined and abandoned, the Guardians have a natural affinity towards haunting relics of the past:
*Many learn to master the stone of ruins - incorporating it, and transforming it into protective plates, or crushing gauntlets. Of course, a stone Guardian is appropriately heavy. Talent and skill is needed to be able to leech enchantments from ruins along with their substnace. It is possible to expel the stone and give up the protection for more mobility.
*A few learn to incorporate metal, and form it both into blades and protective plates. Fewer still learn to fuse with enchanted metal, yet the payoff is well worth the effort. As with stone, so must metal have been used and lost to flow into the Guardians. Especially suited are for example armor whose wearer died inside, forgotten and unburied, or weapons left scattered on battlefields.
As with metal and stone, the Guardians can learn to absorb other substances resonating with the memories of loss, abandonment and despair.
*It is possible for a Guardian to learn to shape crystals, by absorbing minerals from mountain lakes, brittle but sharp and radiant - yet someone must have drowned in that lake, alone, unfound, unmissed.
*High willpower and a keen mind is needed for a Guardian to learn to sheathe himself in fire and lava; they are contained and kept alive by his magic and the magic of the land. Should the Guardian wish to calm his flames, he must rekindle them at a volcano, foundry or fire node then; most important is that someone perished in those flames, watched by others yet his name unknown to them. *Similar it is with lightning: to become charged with the fury of the heavens, it is necessary to ascend a high place, alone with none knowing your whereabouts, be struck by lightning - and to survive.
*Ice is excellent both for camouflage, decent for protection, and can be used to stab or freeze enemies, too. If a Guardian wishes to abandon his icy covering, he must then gather new ice - from a glacier or snow drift where an unknown soul lies buried.
Regardless of his skills, a Guardian also confers one more boon - the Guide is hard to affect with miracles and clerical magic, is rarely seen by oracles or in prophecies, and is overlooked by celestial powers.
The Fleshless have endured the worst of divine wrath, so petty miracles don't compare.
They have been forgotten and have forgotten, so much that Fate itself does not remember.
And their souls - they are near impossible to recognize as Fleshless, yet don't register as anything of creation with the divine flock.
By the very nature of the Joined, working magic of the mind on them is taxing: where the enchanter usually needs to fool but one mind, he is faced with two that show no clear border; and should he probe too deep, the dread hidden memories of the Fleshless may shatter an unprepared mind.
*An Exile Divine: exiled for heresy, the descedants of the Legion are heretical no less, and harbor aberrant and half-forgotten gods along with the idols of their own outcast deities. PCs seeking one of these fallen divinities for aid, information or some more sinister purpose, will have to brave the Wailing Wall and the Joined.
*A Clan Joined: your adversary has returned from the end of the world - with a strange new ally he wears, and the ranks of his kin soon swell with the Joined.
*Under the Searing Eye: a mad god rules over the city of Tharkas, his pervasive mind crushing all resistance - except for the Joined, who are safe from his maddening power.
*Join Me in Life: how will a PC react to a strange entity's proposal to be one forevermore, and adapt to a live-in ally with attitude?
*A False Guide: a mercenary commander has poisoned a village of the Joined - to kill their Guides only, and to set himself up as savior for the disjoined Guardians. The key to his defeat lies in learning the truth and sharing it with the tricked Guardians.
*An Insane Shroud: tramatized by the loss of his Guide, a Guardian fell to madness, and let himself be carried by a fell beast, further increasing the threat of that adversary.
*The Black Cloak Society: a Guardian remembered the ultimate sin of the Fleshless, and the knowledge sent him into the utter depths of depravity. Not only does he utterly dominate the mind of his Guide and forces him to evil, but he is able to convert the unborn into more of his kind, spreading the rule of terror.
Additional Ideas (1)
If you prefer a greater availability, then, for example, it may be possible for just the Guardian to sire a child, so that the mother may wear her child - or for a female Guardian to conceive, birthing the father a young Guardian.