The Door to Darkness is an object from the Kingdom Hearts universe. A trademark of the series, the Door is a tall, white door first seen in Kingdom Hearts in the endless abyss. Beyond the door is Kingdom Hearts, the heart of all worlds, and beyond that is the Realm of Darkness. It is referred to by Ansem, Seeker of Darkness multiple times in the quote "The Door is open!", which is also spoken by Riku when the Destiny Islands are swallowed by the darkness at the beginning of Sora's adventures.
After the battle with the World of Chaos, Ansem opens the door and asks that Kingdom Hearts fill him with the power of darkness. But to his horror, Kingdom Hearts is revealed to be light, which then destroys him. Afterwards, Sora, Donald, Goofy and the restored Riku attempt to close the door, but their efforts are futile. However, King Mickey appears inside the Door of Darknesss, and they manage to close the door. As they do so, the King assures Sora, Donald, and Goofy that there will always be a door to the light, and Sora and Mickey then use their respective Keyblades to seal the Door to Darkness forever.
In Kingdom Hearts Re:Chain of Memories, Ansem, Seeker of Darkness uses his imense dark powers to turn the room of Castle Oblivion's first basement, where Riku and himself fight, into a recreation of the area in front of the Door. Here, the final battle between Riku and the antagonist to gain control of Riku's body takes place. It vanishes when both Ansem and Riku become exhausted from their battle.
In Kingdom Hearts II, the Door to Darkness appeared in Sora's memories when Roxas was dreaming about them. It is also connected to Space Paranoids. Tron needed a password to access the "DTD", and when Sora, Donald, and Goofy returned to Radiant Garden, King Mickey revealed that the "DTD" refers to the Door to Darkness. And since the seven Princesses of Heart were required to open the Door to Darkness, the names of the Princesses were the password. In order to prevent the MCP from accessing the data within, Tron changes the password to the names of Sora, Donald, and Goofy.
Towards the end of the game in The World That Never Was, after Xemnas had absorbed the power of Kingdom Hearts, a similar door appears, allowing access to Kingdom Hearts. Mickey states that the door was indeed a gift from the worlds, who had accepted Sora as their keeper. Sora and the others all enter through the door to face Xemnas, with everyone automatically returning to the Alter of Naught after the battle is over.