Board Thread:Questions and Answers/@comment-2607:FEA8:BD1F:F401:F5E8:BAB1:DD89:27AB-20190925005334/@comment-36170150-20190925035127

Waxer said the benefits for opening the ruins before the war, but I also remember that the developer mentioned that you need to open the ruins before the end of chapter 4 if you want to get the Incubus King Pin (not an equipment, part of the Incubus King key items set we have been collecting), so even if you don't want to open the ruins for the war benefits, you should seriously consider getting it immediatelly after the war if you want to get all the Incubus King items.