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, 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 he and Riku are facing off, into a recreation of where the Door is. Here, the final battle between the Heartless and Riku for control of his 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 needs a password to access the "DTD", and when Sora, Donald, and Goofy return to Radiant Garden, King Mickey reveals 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, but after Xemnas's interference via flying buildings, it is narrowed down to just Sora, Riku, Donald, and Goofy. It is here that the last battles with Xemnas are fought.