That Time I Got Reincarnated as a Slime Novel Volume 21 is written by Fuse with illustrations by Mitz Vah.

Rimuru's disappearance struck the land of Tempest like a violent tremor, but this time, the monsters leading his government quickly regained their cool. They know for certain that Rimuru is still alive, and if he returned to find his nation wrecked, there would be hell to pay. For now, they must deal with the problems in front of them…and so Benimaru shifts his focus to the force of angels led by Deeno. They're invading the labyrinth, and Vega's corrosive abilities have made them a worrisome threat indeed—that is, until two pillars of the dark stand before them…