That Time I Got Reincarnated as a Slime Novel Volume 18 is written by Fuse with illustrations by Mitz Vah.

The seraphim army’s invasion under Michael’s command moves forward, and the Walpurgis Council is held for the eight demon lords of the Octagram to respond to the threat. Veldora’s eldest sister, the true dragon Velzard, has fallen into the enemy’s hands through Michael's skill Ultimate Dominion. Rimuru will need to deploy Tempest's military to various places if he hopes to rectify this situation…