Lady Phosani, or Mother, was a Saradominist Justiciar and member of the Sisterhood. She was stationed in Hallowvale, when the Zamorakian vampyre Lord Drakan and his army attacked it in the Third Age. The kingdom of Hallowvale fell as Queen Efaritay was driven to surrender after her husband's capture. Following the crushing defeat, Phosani felt abandoned by Saradomin, and she and her closest companions, Insela and Otrava decided to desert Saradomin and swore allegiance to Zamorak and Lord Drakan. Phosani and her companions went before Lord Drakan, and Phosani offered her remaining troops as blood tithes and thralls for the vampyres. The sacrifices were accepted by the Drakans, and Phosani and her companions became part of the new regime in the Sanguinesti Region.
Afterwards, Phosani and her companions founded a town just east of the vampyres' new domain in an old hermitage. Here she let the now-Zamorakian Sisterhood develop. Eventually, Otrava discovered a potion called the "Elixir of Everlasting", which supposedly granted eternal life, and as Phosani desired to live as long as her vampyre masters, she drank it. Although it seemed that the potion sustained her, soon the elixir showed the drawback, that those who consumed it would sleep for longer and longer periods, until they remained in a state of eternal slumber. When at last Phosani entered that state, her body was taken into the Sisterhood Sanctuary. The townsfolk were horrified about what happened to Phosani, and dubbed their town "Sleep" (which eventually became Slepe).
At some point after Phosani was taken to the Sanctuary, the Sisterhood decided to seal off the temple, only allowing it to be opened once their Mother wakes up. However, the sealed Sanctuary was eventually opened by Oath Lord D'rows, his Oathkeepers, members of the Sisterhood and the siren Shura, after the Nightmare, a malevolent entity from the Wushanko Isles, invaded the sanctuary to feed on the dreams of Phosani and the people of Slepe, who are suffering from the Sleeper Plague.
With the Sanctuary open, and more adventurers battling the Nightmare, it began to focus on Phosani's mind, generating a stronger nightmare from her dreams. Her coffin is also revealed to be the one directly above the Nightmare's physical body, with Sister Senga tasking adventurers to help Phosani fend off the Nightmare.