incubus is and was (in the grand scheme of things) a nobody. a streetrat who usurped a demiurge maya,(who is now a drunk fat lady) i think she's still hot though.
also he is the weakest demiurge. he maintains some relevance by being something of a powerbroker, he takes over peoples "stories" or lives by implanting his ideals into their heads and granting them power. sort of like gog-agog, actually.
maya and incubus were the students of meti, who was one of the greatest masters of cutting/the sword arts. the only other swordsman who may be her equal is a habitually drunk ex-prince who managed to master the sword by being completely dedicated to acting like a moron. it is unknown whether or not he is alive still.
however, incubus was not truly a student of meti's. only maya was ever to be meti's true student, and incubus was destined to die at maya's hand to teach her about cutting. incubus killed meti and fed her to the dogs, but this is what she actually wanted. incubus is aware he is inferior to maya. sometime during or after his training, he mastered head of john, an internal martial arts style that makes your head completely invulnerable.
he craves violence and war for no reason other than his enjoyment, and formed a bond with jagganoth who he believes shares his passion, however, jagganoth does not take pleasure in his violence, and sees it as a means to an end.
his immortality is either due to his head of john master, or my theory, which is he is also a master of twenty-five purities. all his scars are from his attempts to harm himself after his study of the style