I read a novel in about 1980 about a guy who was sought out by a sentient sword (named Sapia? Sepia?) to continue an ancient feud. The guy was, unbeknownst to himself, a descendent of one warring faction and was being recruited to kill with the sentient sword descendants of the other warring faction. The sword communicated telepathically, but the man could shut the sword out of his mind temporarily when he wanted privacy as if closing a window blind in his mind. Anyone remember that?