Dimensions

The upside down can be entered through the demo portals or through a portal in a lab structure.

The player can only leave the upside down by finding a tree portal through which he can escape.

The upside down has a light and dark cycle.
During the dark cycle, demobats, demodogs & demogorgons can spawn.

When inside the upside down reach y=200 to enter the abyss dimension.

When entering the abyss for the first time, the coordinates of vecna will be posted in the chat (DONT FORGET THEM).

After killing vecna and distancing yourself from the structure he was in, it awakens (mind flayer)

Once the mind flayer is beaten, the player gets teleported to the overworld and demo spawns are disable.

To reverse that use a command /reset_Final_Boss_Kill

The player can also leave the abyss by going down as low as they can which brings them back to the upside down.