What does Primordial essence do?

What does primordial essence do? I have this bar in the center of my screen but I don't know how it fills up, what happens when it's full and what its purpose is.

It is used to switch to a different aspect.

In order to shift into your chosen aspect, you'll have to collect and have enough Primordial Essence to fill its meter, and you can obtain this by killing monsters. Once it is full, press [R], and you'll shift into your apocalyptic form, the meter will then begin to deplete over time and you'll revert back to your original form once it's empty.

This is something that is unlocked at the end of the first chapter.

The first aspect can be unlocked near the end of Chapter 1, you'll be given the opportunity to unlock and choose your first aspect, however, as you progress through the game, you can unlock other aspects and players can switch other aspects at any given time once you've obtained a new one.

Source: https://wolcen.wiki.fextralife.com/Aspects