Commencement

Commencement is the final stage Environment added in the 1.0 Update. It is located atop the Moon of Petrichor V.

The Artifact of Kin will allow it's selected Monster to spawn, but not prevent the Lunar Chimera (Golem) or Lunar Chimera (Wisp) from spawning.

Monsters

 * Lunar Chimera (Golem)
 * Lunar Chimera (Wisp)
 * Mithrix
 * Void Reaver (during detonation sequence)

Cauldrons & Lunar Pods
At the end of the long structure where the path becomes broken, there is an area to either side:

On the left there will be 5 Cauldrons, randomly selected between Uncommon and Legendary. (Denoted by the community as "Soup Island")

On the right there will be 0-5 Lunar Pods, they can be attached to walls and in other unusual places.

Logbook Entry
Like the Hidden Realms, Commencement Logbook Entry is obtained though picking up an Environment Log. Said Environment Log is located within the central hole underneath the Boss' arena and has a time limit of 5 minutes.

Trivia

 * The name "Petrichor V" was originally a fan-name made up by the community. Now it's canonically the planet's name.
 * The Glass Frog Easter Egg can only be found after defeating Mithrix's final phase, during the run-away sequence. It can be found where the player originally spawns into the final stage. This easter egg was requested on the official discord. The log refers to these same "glass frogs", inferring that the frog is more than just an easter egg.