The region was locked up in walls after an inconsiderable amount of trash kept growing. Reborn City tried to clean the area up but trash would come back double the size the next day. They built walls around the region in the hopes that the corruption would stop spreading.
Cain and Aya also lived here with their mother and is the location of the new poison gym.
The seventh gym the player challenges, run by Aya.
The wasteland grotto is accessible after obtaining the HM Strength and changing all the pillars in the wasteland to red. The player will be able to catch a Spiritomb in one of the caverns and will battle a Garbodor in another one.
Note: Similar to the Scraggang battle, these Pokémon cannot be caught
Trees Full of Poison (Bugs)
When you get near certain purple trees, a Venipede will jump out at you. There are an infinite amount of them every time you exit and enter the map.
After you beat Aya, if you go to a certain building while its raining, a guy will say there is a Squirtle playing in a fountain somewhere. This requires HM Strength, the Squirtle can be found frolicking in the fountain during rain, found deep within the Wasteland Grotto. Reenter the ladder of the upper floor to proceed to the next area. Interact with it to battle it. It will know either Mirror Coat, Dragon Pulse, Aura Sphere, or Water Spout as its egg moves.
The Ghost of Kiki
Requires TMX Strength and Rock Smash. The tormenting apparition of Kiki battles the player, which is actually a Misdreavus in disguise. For access, first interact with a grave in front of a building right after going down the first steel beam. Then proceed north through the building and after you come out, use the ledge north. Use Strength on the top-left section of the wasteland to push the boulder to the crevice then head to the middle of the Grotto, go down the steel beam then head up towards the cavern. It will know either Destiny Bond or Nasty Plot as its egg moves.
The player will be able to catch a Spiritomb in the Wasteland Grotto if you have an Odd Keystone. You can get one by mining in glowing rocks (there is a low chance to get one however). It will know either Destiny Bond, Pain Split, Foul Play, or Shadow Sneak as its egg moves.
After you get the Emerald Crystal Keys from Luna, head to the area in Tanzan Mountain where Saphira broke through the wall and gave you the Amethyst Pendant and Ruby Ring. There will be a crystal that is green, interact with it to rock smash through a passage. You'll end up back in Byxbysion Wasteland, and have some puzzles to solve. In the end, there will be a hidden area behind a pillar into a small room containing the Yugeyu Key and another pillar. Interact with the pillar to get a Solosis.