The first thing you have to do is collect 10 Sinistea tokens. When you have 10 tokens, you can only go to the city of Zapapico, to the right of the Paldea Crater. Then you will have to talk to an NPC who is in front of a house on the side of the Pokémon arena.
He will offer to exchange all 10 items for the malicious armor.
Although, as we said above, you can't get the item in Pokémon Scarlet, that doesn't mean you can't get a Ceruledge, as the exclusive version of Pokémon can be acquired by trading with players with the Violet version of the game.
This is all you need to know about how to get malicious armor in Pokémon Scarlet and Violet so we hope we have been as helpful as possible to you.