The legendary Pokémon Regiearth can be found in the Earth Temple, deeply hidden in the forest. However, in order to capture Regiearth, the player must first stop the plans of Team Solar and Team Lunar.
Dullfern Forest is a big forest. It is filled with spider webs that can be cut down with HM01. Each spider web contains a wild Spidox that will attack you. The first Regi-temple of the region, the Earth Temple, is located deep in the forest. Upon your first visit, you can see two Lunar Grunts with Team Lunar Officer Scarlett, talking about their plans in front of the temple. When they spot you, you have to battle them one after another in order to make them leave.