Slumberberry Tea is a mild sedative often used in the Lost Cities. It is said to be a shimmering amethyst color.
Lady Gisela has provided this beverage both to Keefe, when he was mourning and grieving Sophie's "death" in Keeper of the Lost Cities, and to Lord Cassius, who used to take three cups daily.
In Everblaze, Sophie, despite her dislike for sedatives, drank it on Edaline's request because of her ability restrictor; she continued to ask for more of it to hide from reality.
The beverage is made of slumberberries, which grow on bushes. In their scorched form that appeared in Nightfall seem comparable to raisins. The shimmering purple color reminds Sophie of her day in Oblivimyre.