It's non magical, therefore attunement is not required for the effect. Attuning is a process, right? You spend time with the item, maybe fiddle with all the bits, really understand what it does and how it works. If you attempt to undertake that process with this item it will kill you.

Furthermore, that extra damage should really be conditional, you might not always close the demon core on impact. Maybe on an 8 for damage it's such a direct hit that the radiant damage triggers