Question: how to catch uxie in diamond?
Answer: When surfing on the lake, there is a small island with a cave in the middle of the lake, land there. Enter the cave on the land. You will see Azelf, Uxie, or Mesprit depending on which lake you went to (Lake Valor for Azelf, Lake Acuity for Uxie, and Lake Verity for Mesprit.)