Emerikol
Legend
This would never happen in one of my games. I absolutely want the players though to have backstories. I want them to state what sort of character they want to play and then I will help them fit it into the campaign world. If it doesn't fit, we save that idea for another campaign world or we figure out some innovative way to fit it into the world. So the creation of a backstory is something we do together. Often I will run a couple campaigns in a given world before starting a new one. Yeah I like world building. So the second time around the PCs can make a backstory more easily.
An example of an oddball case would be a warforged. Someone wants to play a warforged but I don't have any warforged in my world. So one idea to suggest would be is he willing to be something constructed by a wizard and given sentience. Maybe the PC has no memories before his creation. Maybe behind the scenes the transfer process removed his memories and they might come back. Maybe the wizard is his father who transferred his dying son's soul into the machine. As a DM, I could come up with some ideas on how to make it work.
An example of an oddball case would be a warforged. Someone wants to play a warforged but I don't have any warforged in my world. So one idea to suggest would be is he willing to be something constructed by a wizard and given sentience. Maybe the PC has no memories before his creation. Maybe behind the scenes the transfer process removed his memories and they might come back. Maybe the wizard is his father who transferred his dying son's soul into the machine. As a DM, I could come up with some ideas on how to make it work.