All pieces of the Armor set plus its set tandem weapon, Leontinel's Greatsword can be found from an invading Leontiel encountered. Leontiel can only be encountered after the Boss battle with Starscourge Radahn. As the area of CaelidRegion where Leontiel will invade is their arena and the only way to get there is to fight Radahn's boss battle. From the Starscourge Radahn Site of Grace, head north. Keep heading north till you reach the location of the pillars. Eventually on your way to the stone pillars or under the pillars themselves. Leontiel will send a message to you and their invasion will begin. It does not have a specific spot but it is overall directly north area of the Site of grace. [ ] Leontiel's Armor notes, trivia and tips will be located here.