Duck fat – You will need enough duck fat to cover the duck legs completely, but this will depend on the size of your roasting container, so you might need a little more if there’s more space around the meat.
Once the duck has finished cooking, you’ll find that there are some juices that collect in the bottom of the pan, which don’t cook away because of the oil. You can let the whole thing cool and solidify to help separate the two more easily. Discard or taste it and see if you want to use it for anything else.