Caroenum is a Roman liquid sweetener made by simmering grape must until it has been reduced by ⅓ of its original volume.
Nowadays, substitutes are made in various ways.
You may wish to compare Defritum, Caroenum and Sapa.
Substitutes
Reduce 500 ml of wine to 200ml. Add 2 tablespoon honey, and salt to taste. Note: honey is not original to the manufacture of this ingredient. It’s used to compensate for having started with wine, which is less sweet than must.