Tensei shitara Slime Datta Ken: Coleus no Yume

That Time I Got Reincarnated as a Slime: Visions of Coleus, 転生したらスライムだった件コリウスの夢

The story takes place between the first and second season, where Rimuru confronts the swirling conspiracies in the Coleus kingdom. (Source: MAL News)