Nightmare Shard
A Nightmare Shard, typically, is a chunk of obsidian-like stone, with what appears to be a jagged, organic shape, typically an elongated spindle with high surface irregularity. A typical specimen is 2-5 inches long, and 0.5 to 4 kg in weight (on Earth). Its surface is deep black and highly glossy, with nearly no hue to its reflections. Hyacinth Knight is known to possess one in July of 1987.
Hyacinth's analysis of the shard, using cutting edge tools (for her time) in the labs of Paradox, Inc., have shown the object's outer surface to be comprised of an unknown alloy, principally comprised of an undiscovered element numbering at least 1300 protons per nucleus, with small fractions being comprised of other elements with similarly high atomic numbers. The atoms are arranged in an amorphous structure that is decidedly not organic, but also non-repeating; definitely designed, but by means unavailable to humans, to say nothing of the process of actually constructing it, or even sourcing its raw materials.
Only the outermost skin of the shard is comprised of this alloy--clearly, as if the entire shard were made of the substance, it would be quite unwieldy at an estimated 18.2 kg. The interior is a mystery, defying all attempts to analyze, and not merely because the skin reflects or absorbs light and radiation (although it does both quite a bit). But rather, normal physics seems to end several millimeters into the object.
Hyacinth has, at various points in her "past", shattered her Nightmare Shard. This is generally a nigh-impossible feat; her shard has been known to resist .50 caliber bullets, high explosives, intense radiation, and even once getting dropped into the Grand Canyon from a helicopter. It is not known if a nuclear explosion or ultra-high-speed collision (perhaps via spacecraft) would shatter the shard, but it is known that the sword Midnight is capable of shattering it with relative ease. Having done so multiple times (it's a long story), she has had the opportunity to study the effect of shattering it.
Shattering the Shard
When a Nightmare Shard shatters, invariably, it produces an effect superficially similar to a black hole. A rift in spacetime opens, which appears to immediate observers to be utterly void, "infinitely darker than black", and visibly "hungry". It quickly devours spacetime around it, which to an immediate observer appears like the surroundings of the void being rapidly sucked into the void. The void grows as it devours more material, and proportionately increases its apparent "pull force", eventually overcoming structural stability, and even the binding energy of the Earth's crust. Naturally, this proceeds in an exponential fashion, spreading ever faster until it consumes the entire world, and, inevitably, the Universe.
Crucially, this does not appear to be at all like the behavior of an actual black hole. Although no known scientists have had the opportunity to study the phenomenon with proper measuring devices (or if they have, they've neglected to study their findings), at least one, Hyacinth Knight, has been able to study multiple iterations of the phenomenon from up close. The initial void, in her observations, is never the expected spherical shape of a black hole, but an irregular "tear", which doesn't seem to occupy space so much as replace it. This might simply be the strange reality of tiny black holes, yet to otherwise be observed, but the idea further breaks down when considering that Hawking Radiation should annihilate a black hole of such size before it can absorb enough mass to counteract its own decay. Finally, a true black hole is an object with mass, with no "surface" that can interact with the earth's crust. Thus, such a black hole should immediately plummet toward the Earth's core, at the full acceleration expected of Earth's gravity (~10 meters per second), entirely uninterrupted by the surface of the Earth, it's crust, mantle, etc. And though such a descent would allow a black hole, in theory, to aggregate enough mass to eventually devour the Earth, by all theory, it wouldn't do so at exponential speed, and in fact it would take so long to do so that one scientist's opinion (Hyacinth) is that the mantle would cool before it collapsed, leaving the Earth a strange, hollow shell with a core-mass black hole at its center.
In all likelihood, the event is not a black hole. The leading theory (by the one scientist in the know) is that it is a Vacuum Metastability Event, somehow captured and frozen in time within the outer shell, which, when said shell is shattered, acts according to its nature, rapidly collapsing spacetime into a lower energy state that does not correspond with any known physics. The effect of being "sucked in", per this theory, is actually specific to the observer's reference frame; in actually, spacetime itself is being collapsed "downward" (into a dimension that does not exist in standard physics) in the "direction" of the event (which only makes sense to a local observer; theoretically, observers far enough away wouldn't even be able to see the event before it annihilated them). As their spacetime collapsed, the observed distance between them and the event contracts, until they are forced to "contact" it (which it isn't certain they ever do, as measured from a hypothetical cosmic reference frame).
If such is truly the event's nature, it is undoubtedly capable of annihilating the entire Universe, at least as it is known to current physics. It is unknown--but highly doubtful--that anything meaningful to those occupying this Universe (matter, energy, mathematics, etc) exists in a recognizable form in the state after the event concludes.
It is not known if there is any way to disrupt or cancel a vacuum metastability event, but theory strongly suggests that such a task is impossible within the realm of known physics.