I found a very interesting entry in Volo's guide to All Things Magical the other day - an tripart artifact called the Gatekeeper's Crystal. According to the entry, "The key was held by a strange sentinel of the Outlands, known only as the Gatekeeper, who guarded a nexus of gates to a thousand planes and spheres. The Gatekeeper battled anyone who sought to control the free passage of beings through the nexus with a crystaline artifact that was capable of sundering the most powerful wards, mythals, and other protective incantations." It goes on to detail how a FR lich called the Ice Queen stole the artifact and used it to destroy the city of Myth Ondath. The artifact has numerous great powers (depending on whether one piece, two, or three are possessed), including creating dead magic and wild magic zones, activating and deactivating portals, banishing creatures to their plane(s) of origin, suppressing the magical powers of priests, paladins, rangers, wizards, and bards, nullifying various schools of magic, and bringing down protective magic by converting the energy of said magic into explosive power akin to a huge fireball (this latter power was used against Myth Ondath, obliterating the entire city!)
All this has got me wondering what relationship (if any) there might be between the Gatekeeper and the Lady, and Sigil and the nexus of portals. Could the Lady BE the Gatekeeper? Could the nexus BE Sigil? (The authors of the text are aware of the existence of Sigil, as it is mentioned briefly as a destination elsewhere in the book.) Could the Crystal be used as a weapon against the city?
(P.S. the fact that the artifact has three parts is a nice nod to the Rule-Of-Threes!)
Well whaddaya' know, I know this one too. Heh.
There was actually a set of Forgotten Realms novels published a few years ago now, called the Last Mythal trilogy, which heavily featured the Gatekeeper's Crystal. It was described as a trio of either clear or white crystal points, that I believe held a faint violet light within. When the bases of two or more touched, they would meld together into a single piece, with each point radiating outward similar to the points of an equilateral triangle.
The nexus of gates was posed as a mythal-like demiplane created by one of the pre-Crown War elven nations, which, if I recall correctly, connected to any portal in the world and created new ones that could lead anywhere. And the gatekeeper appeared to be a sentient crystal spire within the demiplane, which was capable of activating or deactivating the portals at will, thus enabling or denying their use to anyone as it saw fit.
When all three pieces were collected, they could be separated and spread out over a certain (relatively small range) area and cause a massive explosion in the space between, although using it in this fashion caused each of the pieces to fly away from whoever held them, and scatter themselves throughout Faerun and the planes. And the whole crystal functioned as the key to allow entry into the nexus. I don't recall if any of its other powers were used in those books.
In the novels, it was used to destroy Hellgate Keep, as well as the nexus of gates at the end.