That Time I Got Reincarnated as a Slime, Vol. 20 (light novel) -

Although Rimuru managed to defeat his archenemy Michael, battles continue to rage across the land, and the fighting between Milim's forces and the Insectors is particularly chaotic. Just when the outcome looks to be in Milim's favor, a beautiful woman of ice and snow appears, drastically altering the tides of the war. She is none other than Velzard, the eldest of the only three True Dragons in existence. Once Velzard overwhelms the situation, encasing everything in ice, Milim begins to go on a rampage--and Rimuru has no choice but to return to the battlefield to stop her. OUT OF THE FRYING PAN AND INTO THE FIREAlthough Rimuru managed to defeat his archenemy Michael, battles continue raging across the land, and the fighting between Milim's forces and the insectors is particularly chaotic. Just when the outcome seems to be in Milim's favor, a beautiful woman of ice and snow appears, drastically shifting the tides of the war. She is none other than Velzard, the eldest of the three True Dragons left in existence. Once Velzard overwhelms the situation, encasing everything in ice, Milim begins to rampage--and Rimuru has no choice but to return to the battlefield to stop her!