The Shadow Realm is a plane of existence adjacent to the 2017 Continuum.
The Shadow Realm is a plane of existence that exists parallel to the 2017 Continuum, similar to the Netherworld in regard to the Prime Universe. When trapped in the Shadow Realm, one wanders a darkened version of reality which one cannot physically affect. Various magical rituals allow communication between the mortal world and the Shadow Realm, though they carry the danger of giving powers to dark spirits such as Tulpas.
When Magica De Spell attempted to disintegrate her animated shadow Lena De Spell in 2018, Lena, thanks to the magical bond between her and Webby Vanderquack's friendship bracelet, managed to hang on to existence, but became trapped in the Shadow Realm. She was eventually rescued from it by Webby and her new friend Violet Sabrewing in Spring of 2019.