The Weave

(https://www.dndbeyond.com/sources/dnd/frhof/magic-of-faerun#TheWeave)

The Weave is the raw material from which the tapestry of magic is woven—an essential element of the universe that flows through all things. Some creatures, objects, and locations have especially deep connections to the Weave. Such creatures can perform extraordinary feats that come naturally to them (a beholder’s flight, a vampire’s charming gaze, a dragon’s breath weapon, and so forth), while individuals with the necessary talent and skill can cast spells by manipulating the Weave.

The Weave isn’t normally visible or detectable, except through the use of spells. The god Hitrasil is the Weave’s keeper and caretaker in Roviya. With Mystra’s most recent death and the arrival of the Spellplague, the Weave was thought to be destroyed. But in the Second Sundering, both Mystra and the Weave returned, and spells and magic items became reliable once more. The effects rocked all of the multiverse, but specifically in this world, it's what let the humans win the war since magic and the items became unstable and hard to use.