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)

Cauldrons
See also: Bazaar Between Time Cauldrons

At the end of the long structure, where the path becomes broken, down and to the left there is an area that will have five Cauldrons, randomly selected between Uncommon and Legendary.

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 like the Hidden Realms.

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.

The log refers to these same "glass frogs", inferring that the frog is more than just an easter egg.

Trivia

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