Adaman has found a treasure, but you need to fulfill his request before he will hand it over.

The Request

Talk to Adaman northwest of Lake ??? in the Crimson Mirelands to accept the request.

Do Battle

Agree to battle Adaman. You must battle his level 64 Leafeon, level 32 Umbreon, and level 32 Vaporeon, all at the same time.

Reward

After you win, you get an Adamant Crystal, which can be used with Dialga. You also get 3 Comet Shards.