The Essential Part

The Essential Part is an esoteric component or property related to The Abyss. Save for Bondrewd and the Abyss Tribe, 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.

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. 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.