A random outhouse, standing in for the one Deku found.
Travelling between the normal world and The Void.
This mysterious outhouse is in the possession of Mugen Kagemaru, and seems to be more than just a plain old outhouse.
Mugen requested that Dêkublast Allosuls go and search for The Outhouse, after hearing that he could find anything asked of him. So far, he has had no luck, but both he and Mugen are confident that he can find it.
Mugen later revealed that once Deku found The Outhouse and input a certain command that he had not yet given him, The Outhouse would travel to The Void, and he would be able to use it to return to the players' world.
It is, in reality, an advanced spaceship capable of traveling through time and outer space, and even to different universes.
It is called the SHARDIS, short for "Silly Happenings And Related Dabblings In Shenanigans"