Perhaps we should consider what Odium said in oathbringer, saying he could take away a fused immortality, so this is something Odium can undo at will (maybe). The fused strike me as some kind of pseudo-shard. the only things that might be able to kill them and keep them dead are weapons that in my opinion could kill a shard(I could be wrong). The only things I can think of being Nightblood, Vyre's knife thing, and whatever killed Adolisium. If you can name something else please do.