itch.io

I don't remember much so I'm not so sure, but I think you just need to teleport the ender pearls toy until you go to the backrooms

If I remember correctly there are 2 ways, you can use the ender beads to make the ender girl cum and there is a small chance she will teleport you to the backrooms (giving you the achievement), or you can force the beads to clip through her stomach (get them fully inside her then use the piston or push them into her stomach from the outside) for a higher chance.  Whichever way you choose just keep repeating it, and you should get it pretty easily.