THE ABYSS, also known as the DARK VOID, is a location exclusive to the OMORI ROUTE in OMORI. It is a shadowy realm that houses many dark secrets behind HEADSPACE's existence and perhaps even some of its former residents.
THE ABYSS is the deepest part of HEADSPACE, and as such is the closest area to BLACK SPACE, and is populated by numerous variants of SOMETHING. Angler fishes are present as well, being the only enemies in THE ABYSS not related to SOMETHING. Skeletons of what seem to be KEL, AUBREY, and HERO can also be found. THE ABYSS also serves as prison for ABBI, one of the three great creatures, who played the role of "the wisest" before being imprisoned there for committing a crime against the dreamer.
THE ABYSS is accessible on the OMORI ROUTE only, when there is ONE DAY LEFT, after OMORI has absorbed SUNNY and reconstructed HEADSPACE. To access THE ABYSS, head to the west end of the UNDERWATER HIGHWAY that links to DEEPER WELL, and speak to the HITCHHIKER. He will lead you to a very short passageway that holds the Z KEY at the very end.
If you have gathered all 26 KEYS to the HANGMAN puzzle (and most probably with the Z KEY as your final KEY), the game will force open the Hangman menu. SOMETHING then will appear and crash the game. Reopen it, and you will find that the passageway has opened up and you can explore THE ABYSS in full.
Combat in THE ABYSS is best avoided. The shadowy creatures that populate it are so unnerving that your companions are paralyzed in FEAR from the start of the battle. Even if you defeat them, you gain nothing from the battle. The shadowy creatures do not even occupy an entry in FOE FACTS!, suggesting their frightening appearances are too much to want to remember, or be reminded of.