Truly Immovable Rod
The Truly Immovable Rod (also called the Universal Stillpoint Rod) is a meme derived from a fa/tg/uy's story about how his dickish DM applied some very dodgy (but accurate if it was the real world and not the D&D world) physics to an Immovable Rod, judging that it remained immutably fixed in the universe while the planet they were on was moving (and thus setting it off, relative to the players, at several thousand miles an hour when activated). The very same fa/tg/uy later used an Immovable Rod as a siege weapon to destroy a castle, pissing off his DM.
It was considered that, since every celestial body is moving very quickly, anybody who placed such an Immovable Rod would never see it again, as their system left the stationary rod floating in the void. If this had happened a lot, there could be any number of Immovable Rods out there, simply biding their time until some unsuspecting and unfortunate planet happens to pass through the same region of space an*CLANG!*
WHAT THE FUCK WAS THAT?
Oddly enough, this was exactly quoted in Lucifer's Hammer, a book written by Larry Niven and Jerry Pournelle. In 1977. When a space station orbiting the Earth was suddenly hit.
Dickish GMs who wish to avoid such shenanigans should utilize a setting that is not based on planets, but instead, on an infinite plane of existence consisting solely of land extending in all four prime directions, and without proper "space" as we know it in this crapfest universe that we type this shit in. Or to use a Ptolemean universe, with a planet in the center of the Universe that doesn't move or spin, and sun, moon, stars and planets orbiting around it. Only then will the Immovable Rod function as advertised, thus avoiding the collision of physics and magic, and consequently, rendering all attempted castle demolitions using this method as invalid schlep comedy and copy-cat antics to be derided and punished with style. A final alternative is to claim it's relative to the nearest celestial body, or anchored to a manasphere a la Shadowrun.
Why this wouldn't work
Einstein's Theory of General Relativity (as well as many of the theories it superseded) tells us that there can never be a true "fixed point" in the universe, as all movement is based on the observer. For example you can cause a mountain to move simply by sailing past it on a moving object. To your perspective, you're sitting still, and the mountain is moving! This also applies to planets and entire star systems.
More information here: https://www.youtube.com/watch?v=9eKc5kgPVrA
Why this would work anyway, but not in the way you would think
Say it's not a "universal stillpoint", but rather a "Worldline Anchor". To apply some really weird physics shennanigans, we can assume that when activated, the Immovable Rod instantly gains Infinite Mass, with only a Finite Space (Say, a two dimensional object), that exerts gravity upon itself to such an extreme that time literally ceases on contact with the rod. Such an object could never be moved, and observers cannot move it either, as all of known physics literally ceases the moment you observe it. At that point to put your hand through it would cause instant spaghettification as your fingers are drawn into a pocket reality where time and space moves normally, but entirely seperate from outside observers.
TL;DR, Really fucking scary, reality bending rod that would behave like a hole in the fabric of reality that stops all time within its area
Maybe Spheres of Annihilation are created when somebody tries to make one of these.
Edit: This is what Black Holes are, then: Immovable Rods
Edit 2: It wouldn't actually be a Black Hole as Black Holes are still affected by gravity, its just an infinite mass object that everything would orbit with it not collapsing into a singularity. In reality such an object would simply destroy the universe as it becomes a big crunch scenario where it eats the universe and itself. In essence it literately cannot exist without destroying the universe. We wouldn't even know it would exist as it expands faster than light.
Edit 3: Black Holes have a finite mass, but their density at the core is theoretically infinite according to General Relativity (or not, according to Quantum Physics, but they hate each others anyway). But yes, from a relativistic point of view, an immovable object would force every object to stand still, by either giving them infinite mass (which means infinite inertia, you won't be able to move them) or stopping time itself everywhere (and possibly putting Road Rollers on top of everyone). So let's just say it's an object capable of canceling any force applied to itself and stay at the same position compared to the nearest celestial body (ie. Earth).
- http://suptg.thisisnotatrueending.com/archive/1709686/ - suptg archive of the original thread.
- https://www.youtube.com/watch?v=9eKc5kgPVrA - a more scientific explanation of immovable objects.