Into the Deepest, Most Unknowable Dungeon manga volume 3 features story and art by Kakeru.

The Deepest, Most Unknowable Dungeon is the deadliest stretch of ground in the new world into which Jean has been reincarnated. Even the top-ranked Holy Saint Slaughterer, the Queen of Darkness, and the Bow King are unable to defeat the demihuman hordes, or conquer the dungeon. But using Earth knowledge from his former life, Jean may just be able to develop the ultimate weapon—an arrow that can pierce any armor—and change the game forever!