Huh. I haven't read anything of Iron Fist beyond the first two trades they published last year, but I was under the impression that the Immortal Weapons were a successive line, like Danny and Orson were? The Bride of Nine Spiders also has several different incarnations, and Orson is said to have killed one of the Immortal Weapons when they tried to take his power as the Iron Fist - Did we find out who that was?
Or are they literally meant to be immortal until someone defeats them and kills them?