Multiple Pokémon have received new evolution methods in Pokémon Legends: Arceus as a result of the removal of trade evolutions. Those that required trading while holding an item have had their evolution methods streamlined so trainers can obtain their powerful evolutions much faster.
Onix, the Rock Snake Pokémon, gained the ability to evolve into Steelix in Pokémon Gold and Silver. Since then, Onix has only been able to evolve into this form by trading it while holding the Metal Coat item. Since Legends: Arceus favors a single-player experience, this trading mechanic has been completely removed, giving players easier access to the Steel and Ground-type Pokémon.
To evolve Onix into Steelix in Hisui, players need only to use the Metal Coat item on an Onix in their party. The Metal Coat can be obtained randomly through space-time distortions, though it isn’t guaranteed to drop. It can be easily purchased at the Trade Post in Jubilife Village at any time for 1,000 Merit Points, which can be accrued through finding Lost Satchels.
Once players use a Metal Coat on an Onix, they will be prompted to evolve it into Steelix. The item will then disappear from the player’s inventory, requiring them to venture for another one should they wish to evolve another Onix into Steelix. The easiest method to obtain another Metal Coat is through the Trade Post, as space-time distortions are not only random but are not guaranteed to have the evolution items needed inside.
Onix can be found in the Crimson Mirelands as rare spawns near the Diamond Settlement, including an Alpha spawn that sometimes appears nearby.