The Nexus is a font of power held deep within The Saddle Mountain. Control over it confers many benefits to spiritual entities, many of which stretch beyond the bounds of mortal understanding. As such, many spirits of all varieties seek to claim it.
For most of pre-history, it was held by the Tuatha Dé Danann, of which Brigid was the most prevalent. When Christianity spread to the north, Saint Brigid converted to Catholicism and passed control of the Nexus to the Archangels Michael, Gabriel, and Raphael. During this time, they built The Monastery on top of an ancient Brigidine holy site in order to guard the primary entrance.
Other entities have held shares of the Nexus, or have been able to access its power; the Faeries hold it as a showing of Nature's immense power, while Demons seek to use it to enter the mortal realm and more easily claim mortal souls. The mortals who live on the Isle of Luachmhar have also used its radiant mana to practice rune-based magic, and it can sometimes cause otherwise ordinary humans to develop sorcerous abilities.