Rising Of The Shield Hero (Manga) Vol 15

With his allies far away, the Shield Hero Naofumi is left alone to defeat the Spirit Tortoise and stop its rampage. Ost, the will of the Spirit Tortoise, claims that her real body has been taken over and the only way to defeat the creature is to infiltrate its body. But the further into the Tortoise he walks, the more danger he encounters, and Naofumi is forced to battle the ultimate mastermind of the rampage. One Peace Books
(W/A/Ca) Aneko Yusagi