(reminded myself that I had an Evernote, and popped it open and found my fish-men solo adventure notes, as well as this, written in May 2015. Probably a way to tie Halthrag Keep and Space Dungeon to Black Powder Black Magic by +Eric Hoffman and +Carl Bussler but I can't recall off the top of my head if it had been released, then)
Hyperidium is described in the scientific literature by Dr. Allan Whipplemark-Smythe of Future Earth 124/4b in the year QE2027. The entry from Encyclopaedia Galactica is abstracted, here.
On Aereth it is known in some places as Bale-Iron and Dweomer-Ore, and Green Orichalc by Alchemists. It has the curious and ominous property that it feeds on ambient magical energy and becomes more dense without decreasing in volume - depending upon conditions it adds mass from some other place and time but retains its shape and form. It is especially common in Thrend, near the site of a meteor-fall that destroyed Halthrag Keep and turned the course of the Wizard Wars long ago (speculatively BE1078 +/- 7)
Any time an offensive spell is cast at a wielder of a Hyperidium weapon, attack and damage rolls (not Deed Dice!) are increased by +1d for the duration of the spell (which may allow, for instance only 1 attack in the case of instantaneous attacks). The wielder may choose to give up this bonus to convert and dissipate the spell energy, whereupon the weapon gives off a baleful and sickly blue-green glow, and the spell result is decreased by one result category OR the wielder gains a Luck point until the next sunrise/resting period, which may take him or her above the natural maximum. In the case of instantaneous non-spell attacks, one attempted strike by the bearer will be affected.
For a wearer of Hyperiridium armor or the bearer of a shield, the effective armor bonus is increased by 1d6 each time an offensive spell is cast at the wearer, which stacks up to the maximum of 6 (so a single spell or multiple spells might drive the AC bonus to the maximum of 6). In the case of instantaneous non-spell magical attacks, the next defended attack will be affected this way. Unfortunately, it is difficult to hide or remain hidden as the armor's evil blue-green sheen radiates tell-tale light and marks it as magical.
Large quantities of this refined metal attracts succubi, wights, and Crystalline Arachnoidae who lust after it. It is refined from meteoric ore and the process requires strong acids; the ore from which it comes gives off a sinister blue-green glow. Much suspicion and bad folklore is attached to it in areas where it is found, and these creatures will always attack or target those who possess this metal (or even quantities of the raw ore) first.
see also: Warp Stone; Greenstone shards; Kryptonite (various colours).