The Hidden Ruler is a mythical Pokémon that doesn't actually appear in the game. There's a myth surrounding its existence, but it's not a real Pokémon that you can catch.
The myth claims that it can be found in the Distortion World and that it's a powerful Ghost/Dragon-type Pokémon. However, it's just a myth.
While you won't get anything for catching a non-existent Pokémon, there are other ways to gain rewards in Pokémon Platinum, like:
* Completing the Pokédex: You'll receive a National Pokédex and other rewards for catching all 493 Pokémon.
* Winning the Pokémon League: You'll become the Champion and earn the Champion's Trophy.
* Completing the side quests: Some quests reward you with rare items, unique moves, or even special Pokémon.
So, while you can't catch the Hidden Ruler, there are plenty of other ways to find valuable rewards in Pokémon Platinum.