Magic Resistance. The fractine has advantage on saving throws against spells and other magical effects.
Scrying Focus. A spellcaster can use the fractine as a substitute focus when casting the scrying spell or similar magic, provided the spellcaster and the fractine are within 5 feet of each other.
Two-Dimensionality. The fractine can occupy another creature’s space and vice versa. It can move through other creatures and objects as if they were difficult terrain, but it takes 5 (1d10) force damage if it ends its turn inside an object.
Unusual Nature. The fractine doesn’t require air, drink, or sleep.
Extradimensional Touch. Melee Spell Attack: +5 to hit, reach 5 ft., one target. Hit: 24 (8d6) force damage.
Imprison. The fractine targets one creature of its size or smaller in its space. The target must succeed on a DC 16 Dexterity saving throw or be imprisoned in a demiplane. While the creature is imprisoned, a distorted image of it can be seen on the fractine’s two-dimensional surface.
The demiplane moves with the fractine, has indestructible and opaque walls, and is only as big as it needs to be to contain the target, which doesn’t suffer from hunger or thirst while imprisoned. No other creature can enter the demiplane, and the fractine can’t be harmed from within the demiplane.
The fractine can imprison only one creature at a time and can release that creature as a bonus action. If the fractine is reduced to 0 hit points, any creature in the fractine’s demiplane is released instantly. A released creature reappears in an unoccupied space as close to the fractine (or where it died) as possible. A creature can leave the demiplane on its own by using magic that enables planar travel, such as the plane shift spell.
Mirrored Damage. In response to being damaged by a creature it can see within 120 feet of itself, the fractine forces that creature to make a DC 16 Constitution saving throw. On a failed save, the creature takes 24 (8d6) force damage. On a successful save, the creature takes half as much damage.
Split. When a Large fractine that has at least 10 hit points remaining takes bludgeoning, piercing, slashing, or thunder damage from any source, it splits into two Medium fractines. The new fractines occupy the space formerly occupied by the original fractine, and each new fractine has hit points equal to half the original’s, rounded down. If the original fractine had a creature trapped in its demiplane, that creature is released when the fractine splits, reappearing in an unoccupied space as close to the new fractines as possible.
Description
A fractine is a bizarre, two-dimensional creature that flies through Wildspace and the Astral Sea, folding and refolding like a piece of origami. When it encounters another creature, it flattens into a plane that resembles a trapezoidal mirror between 9 and 13 feet tall.
Astral explorers who have encountered fractines attest that the creatures are intelligent and often allow themselves to be used as scrying sensors, their reflective surfaces substituting for the mirror needed to cast certain divination spells. While being used in this way, the fractine siphons magical energy from the spellcaster—not enough to cause harm, but enough to make the caster take notice.
A fractine needs light and magical energy to survive. It can draw sustenance from a nearby light source, spellcaster, magic item, or magical effect without causing harm to anyone or anything. A fractine that doesn’t consume light or magical energy for ten days begins to flicker. A day later, it folds in on itself and self-destructs, leaving no trace of itself behind. The destruction of a fractine is accompanied by a loud sound reminiscent of shattering glass.
A fractine attacks by falling on its targets, dealing damage as it passes through them. A fractine can also imprison a creature inside a demiplane contained within its two-dimensional form, the prisoner’s distorted reflection visible in the fractine’s glassy surface.
Bludgeoning, piercing, slashing, and thunder damage can cause a fractine to break into two smaller fractines, each one autonomous and capable of imprisoning creatures. When a fractine dies, it folds in on itself and disappears, releasing any creature trapped inside it.
These are a concern.
Oh, I already have plans.
This is 100% the Crystalline Entity from Star Trek
Have I seen this in a Superman movie? Very cool!
Shout-out to the scary floating triangle (The Five Doctors, Doctor Who)
I’m wonder if the spilt property will also happen with the medium fractine
I suspect not as it's imprison only works on a creature its size or smaller. If it became a small fractine it would be incapable of imprisoning most player characters, robbing it of a powerful ability.
Beware the floating space mirrrors!
I love the idea of coming upon one and unexpectedly freeing an NPC that is trapped inside. This has so many fun possibilities.
It could be a transport too…
Trapped creatures inside still age, but don’t need to eat or drink?
oh man, so many possibilities.
Isn't this just the Phantom Zone prison? And yes, I am gonna use this....
MWAHAHA!
I have a dragon NPC I've been working on, whose "lair" is actually just where he places a permanent gate to a vast library demiplane; a resident Fractine seems like it'd make a great addition as a weird guard that can also serve a symbiotic relationship (as the dragon likes to use scrying to learn things).
Intriguing. It’s a construct but no indication of who made them, why, or how.
I’m not sure if even 360 ft of blindsight is going to help this creature if it’s trying to find a spellcaster in the void of space before it “starves”
These things are neutral? And they have enough intelligence to be used as a scrying device? Oh this could make for a WONDERFUL companion.
I used one of these today in a short respite before the end game scenario of a 24 month campaign. Party level 19. It was meant as a fun diversion, a slight roadblock on their way to a final battle with a god over a celestial throne. They broke it into little pieces. Unfortunately I didn't get to imprison anyone in a demiplane. :(
Can it use Extradimensional Touch on a creature it's Imprisoned?
So technically you could have a fractine inside a fractrine, inside a fractine, inside a fractine, inside a fractine, inside a fractine, inside a fractine
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It depending on how the new Astral Plane lore works
There isn't traditionally any passage of time in the Astral Plane, so it could live forever there, just like the Githyanki.
I'm surprised there are not more people noticing the same reference. I thought the exact same thing. Superman prisons that they let float through space... but with a conscious. Great idea, definitely going to use them.

This is a really cool creature that I think I want to homebrew into a smaller version.
So...every dm reading this is in agreement right? Maximum security prison somewhere with a bunch of these as the cells?
Could it be like a mirror mimic?