Demoness Eris is a Dwarf Demon who once engaged in a wager with Demon Pluto on whose planet would be discovered first by mundane astronomers. She was going to win until her lover Demoness Sharon betrayed her and lead the astronomers to find Pluto first instead of Eris. Pluto locked her away under Castle Roogna which she could not escape until a prince married her. She had been locked under Castle Roogna for over a century.
Over the century, she sent out various messages in hope of finding a prince to rescue and marry her. Her messages in bottles ended up in subterranean rivers where they stayed. Her last note was sticky and attacked to a narrative hook, the same narrative hook which got Jumper. The note was thought to be a prophesy on how to save Xanth and reattach the cable between Xanth's Outernet and Mundania's Internet. When Jumper and his companions found out it was not related to the repair, they were dejected.
Eris and Pluto made another wager on the repair of the cable. When she won, she married Jumper, who had been elected as an honorary Prince of Spiders. She was now free.
Eris became a good friend of Wenda. Eris brought the partners of those transported the petrified reverse wood stump to Humfrey. When Wenda found out she could not have children, Eris showed her the future and all of the children Wenda would adopt.