Possessed Dark Soul

 This card is one of a few that is not available in Yu-Gi-Oh! World Championship 2007 by any means, yet is listed as a Fusion Material Monster in one of the cards in the game.

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This monster's appearance bears a strong resemblance to the creature that Gozaburo Kaiba transforms into in episode 121 in an attempt to purloin Seto Kaiba's body.

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OCG Rulings

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If this card is face-down on the field, you cannot activate its effect.: Effect Monster > Possessed Dark Soul

If "Possessed Dark Soul" has you gain control of a Level 3 or lower monster, then you still retain control of the monster even if the monster's Level is increased to 4 or higher.: After you gain control of a monster because of the effect of "Possessed Dark Soul", does control of it reset if its Level becomes 3 or higher?
Example: If "Possessed Dark Soul" has you gain control of "Sangan" (Level 3), then "Sangan" is not returned to the opponent's control if its Level is increased to 4 because it is equipped with "Synchro Boost

Previously Official Rulings

If the opponent has more Level 3 or lower monsters on the field than you have empty Monster Card Zones after you Tribute this card (the Zone it occupied is now empty), you choose which ones to gain control of and the rest are destroyed. If you choose to destroy "Griggle" or "Ameba" in this manner, their effects do not activate since they never came to your side of the field.

You cannot gain control of face-down monsters with this card's effect, so the opponent may chain "Book of Moon" selecting a Level 3 or lower monster and they will keep control of it.

You maintain control of the monsters until they are removed from the field.