Tensei shitara Slime Datta Ken: Coleus no Yume  

That Time I Got Reincarnated as a Slime: Coleus Dream, 転生したらスライムだった件 コリウスの夢, That Time I Got Reincarnated as a Slime: Visions of Coleus, That Time I Got Reincarnated as a Slime: Coleus' Dream, Moi quand je me réincarne en Slime : Le rêve de Coleus, เกิดใหม่ทั้งทีก็เป็นสไลม์ไปซะแล้ว ฝันแห่งโคลีอัส

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