(A mashup of 3 submissions! Special thanks to @FaeAlchemist, @sofsh, and an anonymous user for your ideas! It was hard putting these together, but I had a lot of fun!)
Gem stared at the dictionary, his expression darkened by frustration. It’d intrigued him to see a book with no title among the unidentified magic items in the back room of the shop. The cover was quite strange, too, decorated with simple designs resembling building blocks and toy trains. Opening it revealed that it was, as previously mentioned, a dictionary, of all things. Page after page of completely non-anomalous word definitions. He tried reading a few of them aloud, flipping through to see if he could spot an unusual entry, skipped to the end to make sure it wasn’t growing new pages or anything like that… nothing. Aside from the incredibly strange cover, it was just a regular dictionary.
After about an hour of puzzling over it, Gem finally decided to give it up for now and move on to some other item. A pair of headphones, sure, why not? He slipped them on, and they instantly began to shift around, the headband portion splitting in two and thinning into tapered points. He felt the ear-covering bits press into him, now stuck over his ears as if by some incredibly powerful suction. It wasn’t an entirely pleasant sensation, but nothing too bad, and he could still hear… in fact, he kinda looked like one of the robots from Cave Story, which he’d normally find really cool. After the dictionary, though, he wasn’t having it.
“Ugh, I am just on fire today, huh…”
Gem struggled with the ear-cover… antenna… things… to no avail. It didn’t help that they were especially sensitive, and amplified the sound of him grabbing them quite unpleasantly. The frustration was really getting to him now. He felt heated. Kinda… kinda feverish, actually. What, was he sick now, too?
He left the Magic Item Shop with his head low and his hands shoved in his pockets. Were his hypersensitive ear-things picking up real giggles from the shoppers, directed at him? Or was he just imagining it? He really didn’t feel good, that much was clear. Had to get home. Maybe he had something that could help him with the headphones there… why was he burning up so much?
Gem looked up, dazed. He’d been walking… well, stumbling, really, for some time now. He had no idea where he was. He was so hot, his fever keeping him from thinking straight. Hadn’t even thought to remove his jacket. Why wasn’t it sweaty? Didn’t matter, it had to come off, he had to get some air on his… skin?
His eyes widened as he stared at his arms. Patches of his skin were smoldering, glowing faintly. It didn’t hurt, but it was hot. His whole body must’ve been getting like this… a theory quickly confirmed as he lifted his shirt to look at his torso. The air brushed against the changed skin and in response it ignited into flame. Dropping the “smoldering” act completely, the flaming parts of his body looked more like some kind of orange, fiery slime. The patches were growing… or rather, it was more like they were trying to. Their shapes fluctuated, but they couldn’t quite seem to claim his entire body.
The newly fed flames caused Gem’s fever to spike, and he lowered his shirt in a hurry. He smelled burning, and coughed, and the coughs were loud in his robot ears, and some of his hair was flaring up into firestuff, and whyyyyyy would this be happening at all, especially without at least making him resistant to the heat, he could barely keep his eyes open—
Realizing that he should probably not die to whatever stupid affliction this was, Gem quickly put his own magic abilities to use and began creating a layer of snow on the ground around him. He dropped to his knees, both to bring himself closer to it and because he’d genuinely lost the ability to stand, and felt his fever diminish as the snow beat back against the heat his body was insistent on generating. There were holes in his shirt, burned open by the flames, but their growth had ceased. His changes weren’t reversing, but at least they weren’t cooking his brain anymore.
He laid down in the snow, no longer needing to worry about setting his surroundings on fire, and thought. He’d only interacted with one magic item, hadn’t he? No. No, that was wrong, actually. There was also the dictionary. The dictionary decorated with toys… toys…
Wordplay. That was it. He’d said “I’m on fire today,” and the book had made it so… or tried to.
He remembered what a friend of his had told him not too long ago: that transforming from a human, a being made of all four elements, into an elemental, a being made of only one element, was exceedingly difficult. No ordinary magic could do such a thing on a whim… and that’s why the book had failed to fully transform him. By coincidence, he’d made the book attempt to change him into a fire elemental, a form far too lofty for its capabilities. But, being a simple enchanted item, it tried anyways, like a computer program trying to take a huge input and then crashing.
Gem thought about these things for a little while, collapsed in a small circle of self-replenishing snow while his body fluctuated between human and elemental, his robotic antennae picking up the relaxing ambience of the place he’d stumbled to in his feverous trance… until his exhausted mind finally found sleep. He’d go home and fix all this when he awoke.