Eel sauce is called Unagi no Tare (うなぎのたれ) in Japanese, and it is a thick and sweetened soy sauce. Traditionally, it is commonly used on grilled eel or different dishes that feature grilled eel such as Unagi Don(Unadon/Unaju) or unagi sushi. This Unagi Sauce from Shibanuma is gluten-free as well.