Board Thread:The Last Sovereign Discussion/@comment-87.58.238.209-20190505130813/@comment-36170150-20190505150121

You get another piece of the incubus king set (key items), which might not be available later on.

The pattern so far has been that you get one piece of the set every chapter through something pretty challenging, chapter 1 was the devil pass extra dungeon, chapter 2 was getting to open the war vaults during the summit, chapter 3 was getting good war results and spending political capital to have Riala get one of the hidden caches. I'd assume the challenge of chapter 4 is getting the ProN to open the dungeon.