Zoro Gets Lost: That Time I Got Reincarnated as a Slime

Zoro and Usopp are taking the Mini-Merry out to sea to go hunt for some fish for Sanji to cook when they end up washing ashore on a mysterious island containing a large and menacing portal. Since they share a combined amount of two brain cells, they enter the portal, causing it to collapse behind them. They are now stuck somewhere, where they meet an unassuming slime.

Cover Credits: https://www.reddit.com/r/OnePiece/comments/u0wfm4/idk_why_people_say_zoro_is_not_a_well_written/