Morgan Kaler explains some of Eris' background to Sir Edward as they ride through her camp. As a teenage girl living in New Jersey, Eris found some old books and managed to teach herself witchcraft. Kaler specifically mentions how Eris was able to make little clay dolls and "put somethin' like life into them."
Eris came to Utah in 1879 and began to try and court favor with the Paiute, the Native Americans who lived in the area. By 1880 she had formed many Paiute around her in a cult by using a mixture of Paiute legend and Christianity. She promised great things to her followers and proved her power by cursing a local deputy who had been cruel to the Paiute.
Thanks to her abilities, Eris was able to access the Paiute land of the dead and steal power from the souls who dwelled there. While there, she kidnapped the spirit of the Paiute shaman Kaipa and placed his spirit into the body of a dead preacher. While holding Kaipa prisoner Eris was able to draw power from him. Eris also managed to kidnap a large group of locals from the nearby mining town of Reidlynne and harvest their spirits as raw energy. She then forced Kaipia to physically weave this energy into a shawl making her even more powerful.
Eris was defeated when Edward Grey shot the body of the dead preacher, freeing Kaipa and the rest of the imprisoned souls. As the shawl fell to pieces on her shoulders a giant spirit with the body of a man and head of a buffalo appeared in the sky and brutally transformed Eris into a dead and twisted tree.
Eris was a very strong witch capable of creating zombies from the bodies of the dead. She was able to raise Adam Glaren as a zombie slave and later dozens of locals from the mining town Reidlynne. Eris was also served by a large stone dog which attempted to kill Sir Edward before being driven off.
After pillaging souls from the Paiute land of the dead as well as the souls of roughly a dozen Christian locals Eris grew even more powerful. While wearing a shawl weaved from the energy of the stolen Christian spirits Eris was able to raise a zombie army and wield lightning as a weapon. However, Eris only had access to this power while she had the spirit of Kaipa the shaman imprisoned. After Kaipa's death, this power left her and Eris became vulnerable to the spirits she had offended.