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May only be held by Giratina, transforms Giratina into Giratina-O. Raises the power of Giratina-O's Dragon and Ghost moves by 20%. Griseous Orb cannot be removed through the use of Trick, Switcheroo, Thief, Covet, or Knock Off.
The Griseous Orb is the only item that can be held by Giratina-O. If Giratina holds any item other than the Griseous Orb, it will stay it its regular forme. In order to change Giratina from its regular forme to its Origin forme, the Griseous Orb must be equipped. No other Pokemon can hold the Griseous Orb.
In terms of usage, the Griseous Orb adds to the already large Base Power of its Dragon and Ghost moves. Adding to the Base Power of attacks such as Dragon Pulse allows Giratina to hit the majority of the Uber metagame harder. Because of the effectiveness of Ghost and Dragon attacks in the Uber tier, the Griseous Orb (and Giratina-O) is quite useful.
The downside to Giratina-O holding the Griseous Orb is that it misses out on the usefulness of other items. Because it is necessary for Giratina-O to hold the Griseous Orb in order to stay in its Origin forme, it cannot hold any other items, limiting its unpredictability in the Uber metagame.