The Shade Isle Campaign
Elfsteel is the common name given to the incredibly strong, lightweight, silvery metal which the elves have long called mythrill. Dwarves favor neither name, instead calling it simply silver-steel in the common tongue and keeping proper Dwarvish name for this precious substance a closely-guarded linguistic secret.
Elfsteel is, by its very nature, powerfully resonant with magical vibrations. For this reason, enchantments placed upon elfsteel weapons and armor tend to remain permanent and do not wear off down through the centuries—or even the millennia. Thus, any truly ancient (i.e. pre-Fall) magical weapons and armor that adventures might find are almost certainly going to be made of purest elfsteel (if not an even rarer metal, like orichalcum or adamantite).
Indeed, if a smith wants a weapon that he’s making to be able to hold onto an enchantment for more than a few decades, pure elfsteel is the ideal choice. But, barring that, a bit of elfsteel alloyed into ordinary steel or bronze will allow a weapon to remain magical for a few centuries, at least. Moreover, the working of elfsteel is such that it can only be done through the labor of (or collaboration between) a master blacksmith and a name-level spell-caster or artificer (a sage, magus, or inventor).
Weapons and armor made from elfsteel (whether pure or alloyed with commoner metals) have a magical +1 bonus to hit and damage or to AC. Weapons and armor made from elfsteel also weigh half normal.