What is in eel sauce?
Components:


sweet -> sugar+mirin (rice wine)
salt -> soy 
savory -> soy+mirin+eel bones


At home, you probably won't be able to manage eel bones boiled down into stock. Ignoring that, it's all a matter of mixing and reducing.

Sugar+soy+mirin, reduce to 1/3, revel in the joy of caramel and salt and sharpness.