The Lyche's Crown

Carl Sargent and Gary Thomas created an Ultima Underworld for the Mystara setting. And then they had to offer the players something to do down there. The authors threw in the Cavern Of Rain Continual as a sensawunda site; also the Warrens for old-school monster-hunting. The last of these campaigns was the Crown of Corruption.

I won't say it was the "greatest" campaign, nor the best-planned. It was however the highest-level campaign, on offer in GAZ13. In any fantasy setting, there is an ancient presence, and ancient banes to go with it - the Elder Evil, as the last 3.5e gamers termed it. in GAZ13, that Crown is what we got. And to be fair, this bane is quite literally Bad To The Bone:

This malefic gold crown is set with four huge rubies, which can be treated as soul crystals (two of 6th, two of 7th level, with 5d10 souls in each). No Radiance spells can be cast from it, however. Once per 48 hours, the Crown can cast a charm monster (saving throw at -4) at one PC and, if the saving throw is failed, the PC will do all he can to put the Crown on — with all the terrible benefits and penalties this implies. The wearer of the Crown gains the following benefits: a natural base AC of -4; complete immunity to all charm, hold, sleep, paralysis, death magic (including disintegration) and gaseous attacks; and the ability to radiate both fear and curse (reverse of bless) within 20 feet (separate saving throws needed). The wearer can also cast animate dead three times per day. The wearer of the Crown at once becomes a Chaotic Undead, subservient to the Crown, but retaining all class-based abilities.

That the bane was buried upriver from Losetrel implies that Losetrel, itself, had not yet been built (and that the Gelbalf were still in the City). Otherwise, there's no damn way anyone would dump such a toxin here. (Although maybe the spiders of Losetrel's cave down river were known.) Its interment further preceded when the two City canals were built.

These gemstones are explicitly likened to soulcrystals, the heart of shadow-elf existence. We deal here with Melkor's Crown in The Silmarillion; with four gemstones in place of three silmarils. What Shallatariel finds, then, might not even be the full crown - it might just be one of the bad stones. It leads him to finding or producing the others, and then he assembles the Crown himself.

Some power-mad lordling of the underworld, long before Telemon's coronation 582 AC, had assembled the like of it. It was done by corrupting soulcrystals. Soulcrystals became mainline shamanism soon after 792 BC. By 448 BC Shadow-Elf society had clearly sorted itself out enough to repel a hobgoblin - orc attack.

Although: just because "soul" crystals weren't mainline in the first centuries doesn't mean the crystals weren't known. To that, the tradition links the Crown not with Celebryl, as we'd expect of the capital; nor even with Felestyr but with a heretic shaman of Porador. The story goes that he looted the City Of The Stars before heading out (p. 18). This is, note, funhouse-image to the Porador migration to New Grunland.

That first corruption belongs to the schism: when Celebryl held the City and the Rafiel cult, and the (nominal) throne; but Porador was increasingly holding the food-supply. Clearly the first Crown failed; I propose it was mostly destroyed. Mostly.

In modern times, should the Crown reëmerge: its undead will infiltrate the nomenklatura of Renathys. The Crown wants more souls, and more crystals; Alfmyr is where to find both. There will also be more activity to Dunedea's west, although some of that will be free-willed. Also here's suggestions for the map.