(Source: https://en.wikipedia.org/wiki/Tesseract )
Few know this, but all oozes, everywhere, are all just parts of one creature. A fourth dimensional being, like Carl Sagan's apple traversing into Flatland, who has only just dipped its toe into our limited universe.
More of it will come, in time. And as it comes it will ignore the limitations of our universe, eschewing walls and vast distances and notions of space.
There's no stopping it. When it enters our universe it forms a sort of projection that we can understand, the forms of oozes that one typically encounters. This projection can be countered with sword and spell; however, the thing producing it cannot. Just as a 2D creature could only carve an infinitesimally small plane out of a 3D one, so to for us and the Gelatinous Tesseract.
Perhaps there is no counter, nothing that can stop it. Or perhaps some spell relying not on space and ones relation in it, but the spirit. Could one perhaps charm the Gelatinous Tesseract? Regardless, its nature does not lend itself to negotiation. It is an ooze (the ooze) after all.
Touching these projections carries risk. It can send a sword or a limb or even an entire whole person flying into the Fourth Dimension, like a 2D person suddenly caught in an updraft, spiraling to a spatial mode they cannot understand. You can end up anywhere.
Anywhere is rarely an ideal place to be.
In bookwyrm-ridden libraries there remain scrolls of ancient wizards who used the Singular Ooze to traverse great distances in an instant, to come to know places near and unreachable with supermathmatical hyper-clarity. They projected their souls into higher dimensions, ignorant of the consequences, and became something other.
The Projections of the Gelatinous Tesseract
1. Green and gelatinous, sweating sterile lubricant.
2. Bubbling radioactive molten lead, crusting at the edges.
3. A black sludge, constantly wetly flopping back and forth.
4. Burnt chocolate pudding, thrown from the pan all over the place.
5. Crystal clear and barely moving. Practically invisible.
6. Perfect spheres suspended in the air. Opaque colored glass of primary colors.
7. Pools of paint mixed but not completely. Smell burns nostrils.
8. Drops of acrid oil-in-water, suspended in a matrix of slimy kelp.
9. A cube within a cube. Apple juice inside green gelatin.
10. Ochre jellyfish with tentacles haphazardly splayed out.
What Happens When You Touch It?
1. Pole/Limb/Thing stretches like entering a black hole, giving no thought to walls.
2. Kicked into the Fourth Dimension. End up nearby. Gain an intimate knowledge of your surroundings, giving no heed to obstacle, wall, or door.
3. Kicked into the Fourth Dimension. End up far away. Teleported to elsewhere on planet.
4. Kicked into the Fourth Dimension. End up far away. Too far away. Most likely floating around in Space.
5. Pole/Limb/Thing disintegrates. Couldn't survive the new spatial arrangements.
6. Pole/Limb/Thing embedded in nearby wall/floor/ceiling/object (d4, in that order). Wasn't able to stick the landing.
7. Pole/Limb/Thing embedded in very far wall/floor/ceiling/object. Connecting thing remains in 4D, now exists in two or more spaces, and acts as a Gelatinous Tesseract conduit.
8-10. Nothing, thank goodness!