Victory Road

Victory Road is a route which leads to Indigo Plateau. You can access it by taking Route 22 and talking to the guard blocking the entrance once you have won 8 Gym Badges.

Solrock and Lunatone
If you catch both Solrock and Lunatone in Victory Road, go to Mysterious Grotto. Go to a Pichu nest, Keep walking around in that Pichu grass, and Deoxys might spawn, hint that it has been encountered is that it will say that your "Lunatone and Solrock has been spotted by something from afar" and "Deoxys attacked!"

Regigigas and Giratina-O
You will be accessed to get these Pokémon from certain people in the Hut or outside the Hut. The one outside gives you Giratina-O if you have Giratina, along with Palkia or Dialga in your party. Inside you can get Regigigas if you have Registeel, Regice, and Regirock in your party.

Elite Four
Elite Four can be accessible in Indigo Plateau. The Elite Four has 4 members: Lorelei, Bruno, Agatha, and Lance. If all your Pokémon faint during one of the trainers, you must restart from the first trainer. After you beat Lance, you come across Champion Gary. After beating Gary, you earn the Champion Badge. You can then find some exclusive Pokemon like Palkia, Dialga, and Giratina.

Pokémon
In Victory Road, the following Pokémon can be sighted, along with roaming Legendaries: