That Time I Got Reincarnated as a Slime, Vol. 9 (light novel) — Fuse

AN EVENT OF MONSTROUS PROPORTIONS!
The Tempest Founder's Festival is in full swing, with Rimuru and all the monsters under him keeping busy setting up the attractions. From live concerts to incredible feats of monster engineering (most notably, the massive dungeon), revelers are sure to have their minds blown. But all eyes are on the main event-the First Tempest Battle Tournament-and who should decide to participate but the Hero himself, Lightspeed Masayuki! What will come of his chance encounter with the demon lord Rimuru?