Okay, I remember a while back that the fanon on adamantium was that it was used to seal away the demons.
I think the adamantium swords just emphasize this idea don't you? Adamantium is obviously a divine metal against demons.
The Demons built massive fortresses/temples when some races unleashed them early on. And these races, knowing the mystical properties of adamantium mined JUST ENOUGH to make these sealing swords before retaking the temples.
The undead are not, in fact, evil. Rather, they are the ancient spirits of the guardians if of these temples, eternally bound to guard the swords sealing the demon's original entrances! If they were EVIL (and thus the servants of the demons) why would they fight people trying to get in? Or why wouldn't they pull up the swords themselves? Any beings INSIDE the temples when found must obviously be guardians.
Slade is the ultimate demonic substance, only demons are able to mine it, using ancient techniques far beyond the kenning of mortals. For any mortal to work it is impossible, and thus it is why the fortresses are made out of Slade, and obviously of demonic origin repurposed into a binding ritual that uses the sword.
Demons, for some reason, are incapable of working LIVING rock, or molten rock, as are most other races. But living rock couldn't be found everywhere, and so the Gods plugged the holes of living rock with adamantium.
Your thoughts guys?