The original "John" (aka John the Cursed, aka Unwilling John, aka John of the Thousand Tasks) must have been a truly terrible person, because he brought upon himself the distinctly vengeful wrath of the original creator of these items, whose name has been lost to time. In essence, this small metal hand mirror summons John the Cursed (usually a 10th level fighter with average stats), from wherever he was, in the middle of whatever he was doing, to help the caster with whatever task he is asked to do.
The current John is a high level fighter, but he’s usually reluctant to do anything right away (his usual opening line is “By the gods, what now?”), and has long ago stopped wearing protection or bringing valuable items (like magic swords) with him, and is as likely to appear wearing a nightgown as he is a fancy party outfit. (When John has been a spellcaster, he has not bothered to memorize or pray for any spells, since he never reliably stays asleep for longer than four hours before being summoned by someone.) Although he has only average characteristics, he has one advantage: he cannot die. (Though he would dearly love to be permanently killed.) He is therefore a very reliable extra blade in battle (at least until the end of the battle), and is excellent for the occasional suicide mission (“jump on that explosive rune, please”). He does not regenerate, however, so he can still be eaten by acid or blown apart by an explosion, after which he vanishes and re-forms back wherever he was summoned from, and his servitude is ended. Temporarily. He may be summoned once per day. Unfortunately for John, there are sixteen of these mirrors, and they’ve all been active for decades.
Note that John doesn’t need to fight. As a person who has been asked to do literally hundreds of tasks, he’s something of an expert on everything...especially dangerous things, like what certain spells do or how tough a strange monster is. He also recognizes key rooms from most of the more notable dungeons. No matter what he is asked, John is 80% likely to know about it...though giving information counts as a task.
The reason John rarely jumps directly into doing anything (including battle) is that he wants to get the request worded properly first. He will do one task--one--and then he’s free to go. The catch to his imprisonment is this: he must complete a thousand tasks, after which he is freed, and he is replaced by the person who just summoned him. Every time he completes a task, roll 3d10/d1000. On a result of 000, this is John’s last task, and the summoner replaces him for what is going to be a pretty miserable time. He’s not averse to doing extra work once he has started, but if you ask him to lift a backpack, he will, and then if you say, “Now put it in the wagon,” he’ll say, “That’s two tasks.” If that’s okay by the summoner, he’ll do the second task, but the GM needs to roll 3d10 again…
Note that John is free to refuse any task that’s too distasteful for him. (Torture, sexual favors, etc.) It’s just that every time he refuses, an extra task is added to his thousand, so he is loath to refuse very much unless it’s truly abominable. [This prevents demented players in your party from using him in unpleasant ways; he has a terrible job, but actual abuse is not part of his story.]
USING THE MIRROR OF SUMMONING JOHN IN YOUR CAMPAIGN
The mirror makes a perfect random drop, since the moment it is used (using the command word on the back), John appears, looking haggard and resentful, and his story should come out naturally. (He knows he is not the first John, but he no longer remembers his original name nor has he kept track of how many tasks he’s done. “Lots.’) If players actually take an interest in his plight, he will tell the party that there are sixteen mirrors--all originally part of one large mirror that originally had the spell cast on it--and he will find peace if only he can hunt them all down and take them for himself. He has managed to obtain 1d6+1 of the mirrors, but the rest still remain to be retrieved. (Though he has an idea where they are, of course, since he’s been summoned by the people who own them and he recognizes a few landmarks.) This could, of course, lead to a mega-campaign where players track down all the remaining mirrors. Though, more likely, players will not want to derail the current campaign for a sixteen-part scavenger hunt. That’s fine, too. If they are kind to him--say, giving him their mirror, or promising not to abuse it and sticking to that promise--then you can simply have the PCs encounter another one of the mirrors in another treasure drop, making John a recurring character ("You guys again!") whose fortunes have risen and/or fallen in between every appearance.
Note that if at any point the GM rolls 000--or if the GM simply decides it would make a fun story--then the summoning character vanishes, John stays with the party...and John has all the mirror parts he’s managed to collect on him. This means that to free their colleague from the same curse, the party needs to hunt down the other mirrors, only this time it’s personal, and they have a head start on the full collection. Over the next several days, John will be shell-shocked, unable to adjust to not hopping all over the place, and amazed that he can suddenly bleed and die. (And he will be more than willing to help the PCs take revenge on theother mirror's owners, since John knows each of them in a professional capacity.)
Of course, the GM may simply reduce the number of mirrors to a manageable five or six if they like.