Parasite Mind is a spell used originally by Zorc Necrophades to transfer a fragment of his soul within the Millennium Ring and later by Yami Bakura to seal a portion of his soul within a piece of the Millennium Puzzle and Thief King Bakura to seal his soul in the Millennium Ring.
Dark Bakura used this technique to seal a portion of his soul into a pair of dice at the final phase against the Dark Yugi in Monster World to create the Doppelganger Dice, in order to ensure that he will roll a double 0 to win. However, it backfired when the normal Bakura used the sealing process to transfer his own entire soul into the dice, causing them to shatter, and Dark Bakura's roll was annulled. This would have killed Bakura, but a portion of his own soul still resided in White Mage Bakura, allowing his revival.
In the manga, when the Millennium Puzzle was smashed by Mr. Clown, Dark Bakura sealed a portion of his soul in one of the pieces. In the anime, he did this when it was smashed by Bandit Keith. Once Yugi successfully reconstructed the Puzzle, Bakura was able to secretly infiltrate the soul room of the pharaoh.
In the Dungeon Dice Monsters game, Yami Bakura uses this ability on the player should they lose to him. It brings them under his soul's control, though this has no bearing on gameplay.