The Deep Magic is the essence of reality. The High Lord sang the world into existence, then sang the seed of the Great Tree into existence, placing it into the world.
As the High Lord was singing the elves into existence as a magical energy field to take care of the Life of the world as represented by the Great Tree, the Dragons appeared from “somewhere else,” and the world became imperfect/out-of-balance. Sorex, the First Dragon, was a being of immense power, drawn to the world by the High Lord’s song.
As the High Lord’s song crafted the Deep Magic, Sorex added its powerful, throaty croon to the music, and the elves fought to syncopate with their own voices, and the seed of the Great Tree sprouted, and Life came into the world. The echoes of the First song drifted through the branches and leaves of the Great Tree, tended by the elves, who had become many as the harmonies of the world’s Deep Magic spawned derivative musics. The dragons, too, became many as the songs reverberated in all the places that dragons are from.
All true elves know the Deep Magic as do all ancient dragons. Adult dragons feel the edicts of the Deep Magic just like their feel heat and cold and gravity, even half elves (eladrin, arch-fey) and quarter elves (high elves, fey) can feel the will of the Great Tree and can occasionally feel the Deep Magic. Eighth elves (wood elves) can feel the Great Tree’s presence, but can only intuit the Weave of the world. Further-distant, part-elves (half-elves) are too removed from their elven heritage to feel the magic of the world unless they are involved in some sort of mystical lives.
A true elf has no form or body and is separate from other true elves only in the notes they sing in the world. They are creatures of essence and magic and creation. They are beings who live outside of time. They were with the Singer when he sang them into the world. They have felt the world as it has changed through time. Their harmonies grow and change and flow and ebb with the world of Erldworn. They are powerful beings.
In order to interact with the world, they can take form, but this dulls their connection with the song of the Deep Magic, and it is unsettling for them to experience the world thus. It is also a trap. The sensations of the physical world can entice an elf into holding a form, and when they grow so accustomed to the physical world that the Deep Magic slips just past them, they lose the ability to ascend to the realm where the song is sung.