Commencement

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

Monsters

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

Easter Egg
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.

There may be more to this easter egg at first glance; The Purity item specifically mentions "chasing glass frogs in the sun"

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.

The Environment Log of this area has a timed limit of 5 minutes after entering the stage, just like the Hidden Realm: Void Fields Log entry.

Trivia

 * The name "Petrichor V" was originally a fan-name made up by the community. Now it's canonically the planet's name.