A marilith is a female-type demon, with the lower body of a snake, and the torso and head of a woman (with six arms). They seem to arise from the Abyss and cause a lot of trouble.
One in particular was being dragged through the Cave of Worlds by twelve knights of Hendrake to be deposited in a garbage universe. Unfortunately, Shizumaru was passing through with Davros, Rimururu, and Michelle at the time, on their way to recover the Seed of Hope. She managed to cast a glamor that made Shizumaru think she was his mother about to be raped by a bunch of slavers--the knights--and in the ensuing fight, he killed ten of the knights and set her free. Davros managed to free Shizumaru of the glamor, and he fled. This, unfortunately, led to a price being placed on his head.
Forty years later, Shizumaru was apprehended by Hendrake and was set to go on trial, accused of murder and possibly facing the death penalty. However, the night before the trial, the marilith attempted to seduce Moriya in the guise of Motoko, in order to take control of his mind and enslave him, for reasons still unclear. Fortunately, the timely arrival of Mitsune under odd circumstances made Moriya realize something was wrong, and with the help of the real Motoko and Anno, they managed to subdue and imprison the marilith. She was forced to testify at Shizumaru's trial under a geas, and will probably finally be cast into that garbage universe as previously intended.
This demoness caused quite a lot of trouble and pain for both Shizumaru and Moriya, but in the end, her capture helped undo at least partially what evil she wrought. Her testimony strongly helped Shizumaru's case, and he was found guilty only of involuntary manslaughter and sentenced to fifty years imprisonment. Moriya, who was undergoing some emotional difficulties because of his unrequited love for Motoko, was further put through the wringer by the near-seduction, but it gave him the impetus to make a real effort to get over Motoko, and this eventually led to the ill-fated romance between him and Mitsune.
More recently, a couple more Mariliths have turned up. First, one which may have been an agent of Ends of the World tried to hack Akari's spikard but was beaten down. A subsequent braineating attempt to figure out the Marilith's goals turned out to be a trap which sucked Akari's soul into the Abyss. Here, she happened upon a shard of Moriya's soul which had managed to possess the body of an incubus shortly after his death, along with his Marilith rescuer and lover who, doubly ironically, has taken on Mitsune's appearance to please him. The two of them helped fight off an attempt by Ends to turn Akari into her puppet and have apparently become Akari's should-have-been-parents in the process (giving her a Marilith form too, now). All three escaped the Abyss via assisted spikardport once things settled down. This is, no doubt, going to create a lot of awkwardness for many people.