Guys I have my tinfoil hat on just warning...
It's a portal to another WORLD. Moonlord is the final boss of the world we're in atm, but that portal leads to a whole new world. New biomes, bosses, etc. Difficulty scale goes up way high and it's like playing from the start. Bosses from here on are super tough and very difficult to defeat. New soundtracks. New "corruption/crimson". Green and black-colored maps maybe? Green could be called the "Vile" and the black could be called the "Abyss".
...Or it could just be a cute lil bunny waiting on the other side that does nothing