Carnwennan is a dagger from Arthurian legend. Might not ring any bells as it and some other talismanic objects get overshadowed by the whole Excalibur bit.
Here's a snippet of Art's holding Carnwennan for the first time. Context: He had to swim to the Other side and travel the secret paths in order to acquire it. Handy blade against manticores and fertility gods, too.
. . . The knife possessed more weight than he'd first assumed--dense with memories of forgotten ages and the ghosts of many hands. The damasked blade whispered its mystery to Art, and he thought of deserts, knew the handle was comparatively new. The white horn bore natural divots like the tally marks of growth from whatever centuries-dead stag had borne it. A few swirls and spirals and triquetrous sigils, though, drew his eyes to their burnt etchings. He felt their concavities move against his palm and impart more grip as though tricking his flesh and hand to maintain tightness. Sucking at them. Forcing conformity. Where blade met hand guard--a short brass piece--black flames licked and rumpled, tentatively seeking, then kneading and encasing Art’s fist. . . .