Uranium

The concept of an imprisoned evil (or mad, or both) god is pretty common in fantasy. D&D has Tharizdun, Pathfinder has Rovagug, the "Wheel of Time" series has The Dark One, and Middle Earth has Morgoth.

Of course, Greek mythology had Uranos. From whom we get the word 'uranium'.

As noted in "The Long Tomorrow", uranium is of course a source of immense power. And it is also a dangerous source - it can be harnessed, but absolutely must be handled with care.

For RPG purposes there's a fairly thick seam of material to mine there. Maybe that's the thing that dwarves are really spending all their time digging for, when they "delve too greedily and too deep". Maybe drow equipment is infused with radiation that renders it magical but extremely dangerous. And, of course, there is also the possibility in a fantasy realm that what is being mined is not merely an element named for an imprisoned deity, but actually the very stuff of that deity itself. In which case, what manner of powers, and what manner of dangers might be unleashed?

(I don't particularly have a conclusion at this point. Just musing on the topic...)