The Bathroom Curse is a way to summon Tails Doll.
Wait until exactly 11:00 PM. Go into a dark bathroom, turn out the lights, and lock yourself in, including all the windows. The bathroom MUST have a mirror, as this is his portal. Play or sing Can You Feel the Sunshine or Living in the City in reverse in the bathroom. Try to sing along as you are playing it. After this, wait a few moments. If you leave, nothing will happen. If you are unlucky, Tails Doll will appear from the mirror and force you into a deal. If you say no, he will kill you. If you are lucky, you can get out in time, though this is rare.
This is another, less deadly result of the Bathroom curse. If all the steps have been followed, Tails Doll himself may not appear, but you may catch a fleeting glimpse of him. In the mirror, a red light, not too different from an angler fish's lure, will appear in the mirror. If any lights are turned on or shone on it, the light will disappear. The red light is the Tails Doll's gem (the Blood Gem) hanging from his head. If you see it, it means the Tails Doll won't come to you, but is instead watching you. Seeing only the Blood Gem is extremely rare, and it means that the Tails Doll may be taking a special interest in you. Seeing the Blood Gem is apparently more common if the ACID Remix is played in reverse as opposed to the normal song.