Ygdrasil

An Ygdrasil is a "braid of worlds", also known as a World Tree(the original meaning of the word, in the mythology of the Northlanders of Azeroth Beta). These are dissimilar to the Soul Braids cultivated by the Angels of Eärendil(and the newly formed Angels of Madoka), and are essentially worlds, universes, where travel is easier between them than between universes outside the Ygdrasil. There are currently seven known Ygdrasils, the first and most well known one being planted by Don Collyer Arnaz though anyone, regardless of power, can plant an Ygdrasil with special techniques of meditation. This is usually fatal, for someone who is not a God Tier in their universe or an angel of Madoka or Eärendil, and people who die planting an Ygdrasil *become* their entire Ygdrasil, experiencing the life of EVERY being within the Ygdrasil they have planted. They have no control or free will after this though - it's like a permanent movie.

Fandoms
Norse Mythology(Yggdrasil as a concept), Warcraft series(Azeroth, and its world), Homestuck(God Tier), Tolkien(Eärendil), Puella Magi Madoka Magica(Madoka). Also vaguely the esoteric Elder Scrolls concept of Amaranth, which inspired the process of creation of Ygdrasils by mortals.