most basic fact of the Thorned One: it can pass through mirrors. To it, the mirrorcatch box isn't solid, it's a tangle of overlapping portals.
So, you reason, when it tried to move through one of the mirrors to reach you, it can't have moved more than an inch or so before running into another mirror, and then it would have passed back into the demon realm. And, if the portals possess the same relative location in the Warp as they do in realspace, maybe it passes through a second portal to appear back in the box while it's back half is still in there, and intersects itself. And then add in that the smallest cube within the box is made of mirrors too small for the snake to pass through - or, at least, for all of the snake to pass through...
So parts of it are obliterated as different parts of it's anatomy tries to exist in the same place as other parts of it, and other parts are sliced neatly off by the edges of portals where they meet entirely different portals, and the portals too small for the snake to fit through. But the snake doesn't just die - it's not that fortunate. Half of it is still within the realm of unreality where demons (you theorize, far beyond the limits of your knowledge) cannot die, and the energies of that place are drawn into it to repair the damage done. But before those energies can become snakeflesh, the part of it that was in the warp has passed through the snarl of mirror-portals into reality, and the raw energy, suddenly subjected to the laws of reality instead of the non-laws of the warp... well, apparently it pours out of the box in the form of what you theorize is raw, liquid Qhaysh. Or at least an untainted and balanced combination of every colour of magic at the same time - isn't that what Qhaysh is? Or is there more to so-called 'high magic' than that? You don't know. You're not sure if any human knows.
And it just keeps on happening, because to the snake there is no solid surface in the box. The parts of it in reality are subject to gravity, but can never stop falling because the closest thing to a bottom is a portal into the warp. It is suspended in an eternal free-fall, forever unable to either heal or die. And in that state it constantly drains magical energies into reality