Regice is a Legendary Pokémon and is not obtained through fossils. Here's how you can get Regice in Pokémon Diamond:
1. Find the Regice in the Stark Mountain: After you defeat the Elite Four and become the Champion, Regice appears in the Stark Mountain. It's on the fourth floor, in a small chamber behind a waterfall.
2. Catch Regice: Regice is a powerful Pokémon, so you will need to use strong Poké Balls and possibly some items to help you catch it.
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