Yxunomei wished to precipitate an ages-old vendetta against the baatezu Belhifet while also returning Icewind Dale to its ancient jungle-like state, wherein her serpentine followers could thrive. Yxunomei arrived on the Prime Material Plane sometime after Belhifet, often simply referred to as "The Old Enemy", was banished from Baator.
In 1281 DR, Arundel, the archdruid of Kuldahar, sent a messenger to the village of Easthaven asking for help in determining what evil plagued the Kuldahar pass. Aid came in the form of a group of adventurers who eventually made their way to the demon's lair, known as Dragon's Eye. Once inside, they destroyed the marilith's army, including the necromancer Presio, before encountering the demon herself, disguised as an odd little girl. Yxunomei refused to give up the Heartstone Gem, an ancient artifact Arundel wished returned to Kuldahar, and was eventually slain by the adventurers.
Later, it was found that the women of Kuldahar had been impregnated by Yxunomei's yuan-ti. They later gave birth to female-only crossbreeds, with yellow eyes and green skin.
Yxunomei operated from a fortified lair beneath a dormant volcano in the Icewind Dale region known as Dragon's Eye. Here she began gathering an army of lizardfolk, Talonites, various undead, and yuan-ti. The necromancer Presio resided on the third level of Dragon's Eye, leading an undead horde and gathering information about Belhifet and his allies.
Yxunomei's actions were, as Belfihet would say, poorly hidden; they included abductions of Kuldahar residents and weather disturbances. She also made her Talonians steal the Heartstone Gem artifact from the Temple of the Forgotten God, which was supposed to be used to reveal Belfihet's location and forces.
When confronted by the heroes of Icewind Dale, she claimed that, in her world, conceptions of man and woman, adult and child had no meaning and did not matter during war. Overall, the form of a little girl served her well, because material plane inhabitants tended to feel sorry for children.
It is not known if Yxunomei knew Belhifet was disguised as Revered Brother Poquelin. According to Presio's war journal, Poquelin was suspected to only be related to the Old Enemy.