Evolution

Like the original games, Pokémon Mystery Dungeon Rescue Team DX locks out evolution in the games until the end game. After you have helped save the world, a cave will open up by Whiscash's Pond and in here, you'll have the ability to evolve your Pokémon

This can only be done here and Pokémon can't evolve in standard dungeons. However, unlike the original games where there were multiple different items to evolve specific Pokémon, Rescue Team DX replaces that with just one item: an Evolution Crystal. Evolution Crystals are typically found towards the end of really hard or long Dungeons or as special rewards for difficult Rescue Missions.