Sekiro: Trail Blazer by Dan Watson
The demonic face from this entry is clearly featured in the endpaper of Soul Arts. However, Trail Blazer by Dan Watson (Website, Instagram) is not in the book.
It did come in at 9th place in VaatiVidya’s art competition New Prosthetic Weapons for Sekiro: Shadows Die Twice.
So my guess is that it was at some point considered for the book and the inclusion in the endpaper stems from that.