Aelf's Breastplate is one of the Holy Relics, the assortment of equipment belonging to Aelf, last of the noble Falshire Knights. It can be found in the dark room that Adam and Rebecca come through just before going up the stairs to Florentine's Observatory.
According to Aelf, the Cuirass, consisting of boiled leather and plate, "will prove sovereign against both spear and dagger; against the ungodly, it will shield the heart and ward the soul."
Adam: This looks like something Aelf would wear.
Adam: Aelf's breastplate, his cuirass. Makes me feel somehow... I'm not sure. I feel ready to take on anything.