Slayer of Secrets's Chest Slayer of Secrets's Rucksack Slayer of Secrets's Satchel That spelling can't be right. Shouldn't it be just Secrets' (no extra "S")?
I think it can go either way, depending on style guide: https://blog.apastyle.org/apastyle/2013/06/forming-possessives-with-singular-names.html
It doesn't help the cause when you use all caps ... and spell achievement wrong. It's a style thing that can be correct, depending on which guide you choose (no official English academy for language ... thank goodness). Cleaning up the spelling errors (or obvious grammar errors) is great. I don't think this is one of those. BTW, spelling errors I usually send in via /bug and they have occasionally been fixed (not sure if it was my reporting or someone else).
Secrets (plural) of Faydwer is the expansion, Secrets's (posessive of Secrets) is the bag belonging to Secrets.