Unagi sauce, or Eel sauce, is a reduction of eel bone broth, soy sauce, rice wine and sugar. It is first made as the base sauce and then continuously flavored by dipping grilled eel into the sauce, which is part of the grilled eel preparation process.
Unagi sauce is rarely made at home. Some eel restaurants have been using the same sauce mixture for over a hundred years and keep adding the base sauce every day. Although they do not taste like a completed eel sauce, there are many eel sauces sold in the markets which taste like the base sauce.
Grill the eel bones until they are brown. Place them in simmering ricewine and add soy sauce and sugar. Reduce the mixture until it becomes asyrup. Discard the bones.