Imagine you are in a deep, dark dimension with no vegetation and no exit. The earth as we know it has sunk into chaos through human exploitation. What remains is this terribly dark and endless cave. All civilizations are long gone. Dangers lurk around every corner. Evil creatures are waiting in the darkness to tear you apart. Light is a thousand times more valuable than all gold and all diamonds in the world. But the shining light of the sky cannot be seen in this endless night. Loaded with only a few seeds and saplings, your task is now to uncover the mystery of this dimension and to break the power of darkness. The future and the hope of all humanity rests on your shoulders! Good luck, brave traveler!
You can return to the overworld by using the teleporter block. Place it down and right-click it and you'll get transferred back to the overworld. You can either craft the teleporter (which might be impossible, since you can't get netherite and a nether star in the deep dark), or you have to give the teleporter to you via cheats (you find it in the Miscellaneous tab in the creative inventory).
If for some weird reason this does not work, you can type in your command line:
/execute in minecraft:overworld run tp x y z
This will teleport you to some coordinates x, y and z in the overworld. This is the ultimate last possibility and you should only do this, if the teleporter does not work for you.
It's also important to note, that there's a config file in your config folder named "ddd-common.toml" that contains a bunch of parameters. Somewhere you can find a line
teleporter_block_included = true
By default, this value should be set to true and it means, that your starter base (if enabled) has a teleporter block to teleport you back. Changing this value to "false" will prevent the mod from adding the teleporter to your starter base. This might be useful, if you don't want the player to be able to teleport back to the overworld anytime. But keep in mind that it might be impossible to craft the teleporter, since some items aren't available in the deep dark!
Mineuniverse