From Bulbapedia, the community-driven Pokémon encyclopedia.
Chroma Ruins (Japanese: ヌリエのいせき Nurie Ruins) is a location in Pokémon Ranger: Shadows of Almia. The player and Keith, just after being promoted to Top Rangers, are ordered to the ruins to meet up with Sven and help him with a Mission. Altru Ruins can only be accessed by traveling all the way through the ruins. The player will find another grunt with a Kangaskhan here, but can not be found in the Ruins. To travel through the ruins, the player must employ the use of Drifblim, Electabuzz, and Sandshrew.
On a certain quest, the player will capture a Misdreavus. Once caught, it will join as a Partner Pokémon. The player can obtain Registeel by using a Level 5 Cut Pokémon (Gallade) on the statue near the exit or the ruins, this being one of three locations of a Legendary titan. The boss Pokémon at the end of the ruins is Spiritomb, but can only be caught during the mission, and does not appear again (excluding the Capture Arena.)
To obtain access to the ruins, the player must use the field move Tackle 3 on the stone slab covering the entrance.