Weave

The weave is the fifth element, and one constituting everything in Toriel, it is the element thanks to which magic exists. When a spellcaster extends its wand and whispers an incantation, he bends and disturbs the flow of the weave to create a magical effect.
As an ever-present element of this world, it influences everything, slowly infusing it with magic. Nature itself is infused with this power, this relation is neither good nor bad, it just is. The flora and creatures evolving with these influences, growing sometimes stronger, sometimes more intelligent, sometimes with magic ability, the effects are many.
Man discovered magic firstly thanks to inherent magic ability, sorcerer, and the like. Those people, gifted with a particular ability to manipulate the weave at birth were the first spellcaster. Soon, understanding this kind of power would be useful, they were studied, used. Thankfully, even if the ability was inherent to some, it was possible to learn to master the weave. Elves, humans, and other races alike had similar learning processes for this and developed different kinds of magic. A well-known example is the elven high magic.
Magic in the world of Toriel is incredibly powerful, the weave as the ability to create life. The ability to create golem and semi-living creatures alike was well known, but warforged, fully sentient, and independent being, are also magical creations. Due to its presence everywhere, magic is tied to all domains, including Science and Religion. Gods influence the words thanks to the weave, and most technological creations work, at least partially, thanks to the weave.
The Various Research, and thesis done on the weave, are the work of the academy of magic, responsible for the various schools teaching spellcasting through the world, they also invest a lot of time and money in research on the subject. But to this day, no one fully masters the weave. for example, there is no recorded instance of 10th level spell or higher, even if it is believed that the great extinction was caused by something of this power level.

Origin

The god of magic, Malygos is the keep of a very special structure called the nexus, his lair surrounding the structure, and the god protecting it, it is the source of the weave in Toriel. But, the god, always eager for magic knowledge, accumulated a large quantity of magical power in the nexus thanks to his artifact, the Arcanomicon, a book containing all the spell that exists. As a resulst of this accumulation of power, the nexus, a well for the weave into this world, grew stronger and stronger. The consequences had been violent, dangerous, and deadly. The world's crust had splintered, and the resulting unstable rifts had torn the very fabric of the astral plane. This made the weave flow in incredible amount on the material plane, making everything partially magical, the plant, the beast, the monster, many become blessed by the weave.
The nexus is on the mythical island of Coldarra, a floating island to the north, inaccessible without the ability to fly, and hidden from unwanted eyes.