Larian Studios Forums Baldur's Gate III Baldur's Gate III - General Inaccessible area in Mind Flayer Colony

That area is the corpse disposal right underneath the prison.

There are numerous ways to get there, all of its entrances are within Moonrise Prison though;

• Either jump through the hole located in cell #3 (your cell if you get caught)
• Or jump through the bloody ritual hole behind Warden's locked office door
• Or simply go behind the walls of the cells and descend to that area
• Get captured by the Absolute army if you decide to press on to Baldur's Gate too early
• Get captured by the Absolute hiding inside the cracked tower wall

The Illithid Oubliette (the area you are currently in) can only be accessed during the ACT's finale and is not directly connected to that corpse disposal area, so you'll have to deal with Ketheric first and then go back to Moonrise Prison to explore it.