Main Storyline Legendaries:
* Dialga: The Temporal Pokemon, the mascot of Pokemon Diamond. You'll face it at the Spear Pillar.
* Palkia: The Spatial Pokemon, the mascot of Pokemon Pearl. While not obtainable in Diamond through standard gameplay, it's a valuable trade partner from Pearl.
Other Legendaries:
* Cresselia: The Lunar Pokemon, residing in the Fullmoon Island. You'll need to complete the post-game story to obtain it.
* Azelf: The Knowledge Pokemon, one of the Lake Guardians. It can be found at Lake Acuity after completing the main game.
* Mesprit: The Emotion Pokemon, another Lake Guardian. It can be found at Lake Verity after completing the main game.
* Uxie: The Will Pokemon, the last of the Lake Guardians. It can be found at Lake Valor after completing the main game.
* Heatran: The Lava Pokemon, found in Stark Mountain. It can be obtained after completing the main game.
* Regigigas: The Colossal Pokemon, residing in Snowpoint Temple. It can be found after completing the main game.
* Giratina: The Renegade Pokemon, existing in the Distortion World. You can encounter it in the post-game story.
Note: Some of these legendary Pokemon require certain steps or events to be triggered before they can be encountered. Make sure you check online guides for specific details on how to find each one!