Atlantica is a place you can get to in BLACKSOULS if you’re looking for Little Mermaid and the Poseidon Hotel. Both of those are located there and it’s an important area that holds key points that’ll allow you to finish the game.
To get into Atlantica you first need to find an invitation that’s in the form of a letter in the Snow White Castle. You can find it on the upper floor, in the room to the left. It doesn’t matter whether Snow White is defeated or not, as the letter is not frozen and can be picked up early on.
Then after you get the Invitation to Atlantica, head to the Empty Beach, which is an area you can get to through the Skeleton Maze.
Go to the bottom of the Empty Beach and then step into the conveniently placed hexagram and initiate the invitation! And then you will find yourself in a whole other area, that leads straight to Atlantica.
