転生したらスライムだった件 [Tensei Shitara Slime Datta Ken]

This long-running manga series from Kodansha, launched in 2015, follows a salaryman reincarnated as a lowly slime in a fantasy world, where his unique abilities and diplomatic nature gradually forge a diverse nation of monsters. Written and illustrated by Fuse, the series has become a cornerstone of the isekai genre, celebrated for its inventive world-building, political intrigue, and the charming, unexpectedly powerful journey of its protagonist.