That Time I Got Reincarnated as a Slime Novel Volume 15 is written by Fuse with illustrations by Mitz Vah.

After a fierce battle, Veldora has fallen into the enemy's clutches, and Rimuru is furious. The order comes down to unleash a massacre, and Benimaru and the other Tempest executives couldn't be more thrilled. The fuse has been lit for the final battle between Tempest and the Eastern Empire. But if he wants to save Veldora, Rimuru will need to evolve even further. It's time to release the power he's been holding back!