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Deals damage and has a critical hit rate one stage higher than normal. Assuming there are no other critical hit modifiers in play, this move has a 1/8 chance to get a critical hit instead of a 1/16 chance.
Unlike Roar of Time, Dialga's pitiful excuse for a signature move, Spacial Rend is quite useful. The step up in Base Power from Dragon Pulse to Spacial Rend is small, but helps Palkia a great deal in ripping through your opponent's Pokemon. The low PP can hurt in a metagame jam-packed with Pressure users, but this doesn't ruin the move in the slightest. Feel free to throw Spacial Rend onto any Palkia set for nice, reliable STAB.
Dragon Pulse has a lower Base Power but higher PP and accuracy.
Draco Meteor has a higher Base Power, but lowers the user's Special Attack by two stages after use.