The Essential Part

The Essential Part or Essence is an esoteric component or property related to The Abyss. Save for Bondrewd, the Abyss Tribe, and perhaps Doni, knowledge of its existence is all but unknown.

Both Reg’s Incinerator and the Star Compass have some connection to the Essential Part, each having been called the “light of the Essential Part” and “shape of the Essential Part”, respectively. Furthermore, the soul inhabiting Lyza's White Whistle, Doni, indicated that someone associated with Reg was waiting at the Ring of the Essence.

The term has also been used in reference to the Absolute Boundary by Bondrewd, prefacing it as "The pivotal area of the Netherworld" (の).

Description
Not much is known about the Essential Part itself. However, its mere shape has been referred to as a “precious thing” and its light has drawn comparison to one of the most destructive forces in the Abyss. One of the most mysterious features of the 7th Layer, Ring of the Essence, appears to have drawn its title from it. Given its very name and the reverence shown for it, it is assumed to be quite important.

Trivia

 * In the official English translation of Chapter 35, Bondrewd refers to the Incinerator as "the light of essence". However, when Made in Abyss Movie 3: Dawn of the Deep Soul was released, the translation was changed to "light of the essential part", matching the English translation of "form of the essential part" from Chapter 49.