Board Thread:Questions and Answers/@comment-107.15.49.35-20191007140413/@comment-2601:586:4B00:2030:35A0:7F08:C6D1:831B-20191007150217

You can find them in special optional content that's easily missable.

Incubus King Armor is found in the Devils Path between Stineford and Yhilin, you must get it the first time you pass by or else its gone.

Incubus King Helm is gained by passing the War Vaults resolution in the Ardford Summit successfully.

Incubus King Shield is from investing in Riala's Cache Expedition after the Third Arclent War is ended.

Incubus King Pin is from the New Givini Ruins.

Each piece can be lost forever if you don't get them the moment they're avilable pretty much.