Inspired by this, this, and this. Here is an excerpt from Clark Ashton Smith's "The Beast of Averoigne."
"Now, when the priest and the marshal had gone, I spent several hours in making ready for my journey. It was necessary, above all other things, to compound a certain rare powder that had been recommended by the demon in the purple gem; for only by the casting of this powder could the Beast be driven away before its time. The ingredients of the powder were named in the Book of Eibon, that manual written by an old Hyper-borean wizard, who in his day had dealt with ultra-mundane spirits akin to the demon of the comet; and had also been the owner of the ring...
Then, an hour before sunrise, when the shadow of the great pine had reached the wall and was climbing toward Theophile's window, there came the thing which I had anticipated. Very suddenly it came, and with no warning of its nearness, a horror of hellish red light, swift as a kindling, windblown flame, that leapt from the forest gloom and sprang upon us where we stood stiff and weary from our night-long vigil.
One of the men-at-arms was borne to the ground, and I saw above him, in a floating redness as of blood, the black and serpentine form of the Beast. A round and snakish head, without ears or nose, was tearing at the man's armor with sharp innumerable teeth, and I heard the teeth grate and clash on the linked iron as I stepped forward and flung the powder of Eibon at the Beast. The second man-at-arms, undaunted, would have assailed it with his spear, but this I forbade.
The floating powder, fine as a dust of mummia, seemed to dim the bloody light as it fell; and the Beast relinquished the fallen man, writhing away like a burnt serpent from the fire. Its members and body were loathfully convulsed; and the thing seemed to change horribly beneath our gaze, undergoing an incredible metamorphosis. Moment by moment it took on the wavering similitude of man, like a werewolf that returns from his beasthood; and the red light grew dimmer, and the unclean blackness of its flesh appeared to flow and swirl, assuming the weft of cloth, and becoming the folds of a dark robe and cowl such as are worn by the Benedictines. Then, from the cowl, a face began to peer, glimmering pale and thin in the shadow; and the thing covered its face with sooty claws that were turning into hands, and shrank away from me as I pressed upon it, sprinkling it with the remainder of the powder.
Now I had driven it against the abbey wall; and there, with a wild, despairing cry that was half-human, half-demoniac, the thing turned from me and clawed frantically at the gray stone as if it would climb toward the abbot's window in that monstrous fashion that had been its wont. Almost, for a breath, it seemed to run upward, hanging to the wall like a bat or a great beetle. But the change had progressed too far, and it dropped back in the shadow of a pine, and tottering strangely as if with sudden mortal weakness, fell to the ground and lay huddled in its monkish garments like a black night-bird with broken vans."
Obviously there can be some problems transferring this kind of thing over to a D&D type game, but in a perfect "worldgame" this is what magic would look like.