You can find the Carian Knight's Sword in Liurnia of the Lakes. It will be in a chest in the back of a carriage being pulled by two trolls. It is northwest of the lake.

The Cleanrot Knight’s Sword is a Thrusting Sword in Elden Ring that scales with Strength and Dexterity. The weapon comes equipped by default with the Impaling Thrust Skill. The Cleanrot Knight ...