The Keyhole

In the Kingdom Hearts series each world has a keyhole that leads to its heart. A driving element of the first game is sealing a world's heart by locking that keyhole, which keeps the Heartless from destroying the world; Keyblades can lock and unlock keyholes and doors. Maleficent kidnaps the Princesses of Heart because, brought together, they can summon the Keyhole to Kingdom Hearts itself. Dream Drop Distance adds Sleeping Keyholes as its way between worlds, and the dark beings that inhabit the Sleeping worlds are compelled to find those worlds' keyholes.