Whilst reading the Mary Sue and Have you been overshadowed by some gm contrivance threads, it occurred to me that people do tend to have stories about that one gmpc that irritated the crap out of them.
So, thus the question I ask for fun!
What is the backstory/justification given to you by your gm that is WHY said person is 'just better than you'?
Descended from or is a god/goddess? Ancient king/queen? Or true heir to great power? Super artifact? Something else?
Okay, here's my Mary Sue/DMPC horror story. . .
The setting is a homebrew setting
very loosely inspired by the Pern novels. The idea being that thousands of years ago several Spelljamming ships crashed on some distant world, and the settlers had no way home. On this world, Divine Magic is all but unknown (a tiny order of Elven druids that has next to no contact with humans is pretty much the only divine magic, and the humans know of them only in legend), Arcane magic is rare and secretive, and Psionics is the main supernatural power.
The DMPC is a little girl, she's supposed to be like 12 or 13. She is supposed to be incredibly, inhumanly intelligent, wise and charismatic (INT, WIS and CHA of 18 in 2e terms, this was a 2e campaign), with truly prodigious psionic power (psionic powers as if she was somewhere in the mid-teens levels at least, essentially she had pretty much whatever power the DM wanted her to have, which always worked even under the old 2e system where you had to roll to activate powers).
Why was she like this? Well, apparently the ancient ancestors turned to magical genetic engineering to enhance psionic potential in the humans of the world (the only humanoid races were native elves, humans descended from the crashed spelljammers, and some Orcs and Drow off in the woods, of which Elves and Humans were the only ones eligible for PC's). This meant that many humans had some kind of psionic talent (a substantial bonus to the roll for psionic wild talent), however she was some kind of genetic manifestation of everything they were working for, the inherent full manifestation of the human mind to its potential.
This meant that the DM could have her teleport us around at will, blow up monsters, and generally do whatever he wanted, and whenever we got upset the DM would try to act like a cute little 12 year old girl and whine and be flighty, and all the NPCs were utterly enthralled with how cute and adorable and wise and brilliant and talented she was.
The DM used this DMPC to lead us around by the nose, keep us from going "off the tracks" or jumping ahead in the plot if we figured out what was happening to early, and let the DM enjoy acting like a pre-teen girl for most of the game session (it was a little creepy).
Then at the end of the campaign we find a huge crystalline artifact which shuts out all non-druidic divine magic from the entire world (the campaign plot featured the arrival of Clerics bringing their deities via planewalking and the societal/political/cultural strain this brought, bringing religion and divine magic to an atheistic psionic world.) Despite most of the PC's converting to these new religions, the little Mary Sue activates the relic to seal out all gods forever and strip all clerics and paladins of their powers because no matter how much the PC's thought it was a Bad Idea, this was the way the DM wanted the campaign to go (and none of the PC's with psionic powers who could activate the artifact would, in fact one tried to outright destroy it).