The 3rd Age of Omerün
A sharp-witted shouty robot
Mark V was a mindless killing robot. During an assault, he fell through to a hidden crypt/study. He tried to rejoin his force, but couldn’t—he was trapped.
For the first year, he raged against the walls and floor, trying to dig himself out.
For the second year, he practiced his combat protocols, trying to stay in top form to rejoin the battle (which must surely be ongoing, right? He’s losing his robo mind slightly at this point).
For the third through fifth years, he obsessively cataloged the contents of the crypt, finding that it was full of histories and novels, describing all manner of histories and adventure. He stopped being a raging killbot and started yearning for escape, any new experience. He dreamed of founding an adventuring guild, something that supports its members and shares its exploits—like a friendlier version of the pirate kings in his novels.
Finally, some adventurers (other player characters?) broke into the crypt and found him, reading his favorite book for the hundredth time. Creaking from missed maintenance, he presented them with the treasures of the crypt, and asked to join them.