Registeel

Registeel is a Legendary Pokémon introduced in Pokémon Ruby/Sapphire (2002). It is one of the "Legendary Titans" alongside Regirock and Regice, created by the Mythical Pokémon Regigigas. Registeel is said to be composed of an alloy harder than any metal, with a body forged in the intense heat of a volcanic eruption. It is often depicted as a guardian of ancient metallic ruins.

Players must first have Regigigas in their party.

Access the Iron Ruins located in the southern part of Sinnoh.

Interact with the altar inside the ruins to summon Registeel.
Registeel is a single encounter and cannot be found through standard gameplay, making it a rare and sought-after Legendary Pokémon.