The relic works specifically on the target being soulless. That's why it works on demons. It could, hypothetically, work on the Seven since they don't have "souls of their own," but we don't quite know how they get stuck in the artifact. The champ getting put in happens off-screen, so to say, and we only know that Tollus entered the prism with Kassyra when bringing her in. And even then that's a bad end, so we can't know any of the mechanics behind the prism. And since the Seven have immense levels of power, to the point what Mallach seems to freeze time when you speak to him, and they seem to teleport wherever they want, whenever they want, not to mention having been the ones to create the anti-wraith tools, they likely have knowledge of what they do (preventing a surprise factor that could have been needed to effectively use something like the prism) and could potentially just counter any attempt to use the tools. Plus, if someone uses the stuff and succeeds on taking out one of the Seven, then the others may learn of it pretty quickly and treat them as a high-end threat to directly combat rather than underestimating them like Nareeva did. Also, how would we know that all the relics are in working condition? Sorra's Succor was an artifact from the Godswar too, but there seems to be no magical energy on it, and while that could be because it was never magical, it could also have had an enchantment fade or been damaged to the point that the magic left it.