Minecraft – How to leave Erebus

Unfortunately i don't think there is a way out without following the corridor back the way you came, I'm still trying to figure this one out.

If you're really desperate though you could try opening the game to LAN with cheats enabled, typing /gamerule keepInventory true followed by /kill to try respawning at the spawn point along with all your items (the gamerule command stops you losing your items on death).

Unfortunately these worlds have a habit of changing your spawn point when you're inside the other dimension so you might be a touch screwed. I seem to remember spawning in some obsidian and building a portal to the nether though, Once i was inside i was able to build another portal and go through which generated an exit on the overworld (regular world) and i was able to escape. Only catch was i had to spawn in the actual Portal item that makes up the purple stuff on the inside /give playername 90

Give those a shot see how you get on :)

Youwill need acces to the code to do that and change the terrain generation code yourself. This can therefor only be done if the mod is open source. You also will need to have a bit of java experience.