Within all humans touched by Mamono mana rests a darkness, which seeks to burst forth, engulfing the body and soul.
While some view this darkness as a glorious bedrock to be worshipped, and others consider it a profane sludge to be purged from oneself, it accumulates regardless, warping women into monsters, and men into incubi.
Women who cannot cope with their monstrous forms, or who have been disfigured by injury, seek solace in the arms of Rosaria, mother of rebirth.
In exchange for an offering of tongues, stripped from animals or assailants, Rosaria allows others to restore their forms to a human appearance, and alters their mana, so as to allow them to live among humans despite their nature as monsters.
(See Alp and Dhampir page, which set precedent for such.)
Those who perform this rite will usually return to their families, or communities, often relieved or elated.
But for some, the allure of altering one's appearance is too great, and urged on by their pride and vanity, they will continue to carve tongues from animals in order to alter themselves further.
Fittingly, those whose greed for transfiguration knows no limits are eventually transformed into these eerily beautiful grubs, their bodies forever living, and any attempts at further changes rendered impossible.
Not posting it on the site, to appease our fandom overlords.