Encounters with tieflings are few and far between, even rarer than encounters with Pandarens and Drow.
Not much is known of this race beyond them being humanoid yet demon kin. Any records of such encounters and dealings with them are kept by mages and clergy members of churches - those that can handle one form of magic or another. These records are kept highly secret and only the most privileged have access to them.
What is known and shared - if the knowledge is essential - is that tieflings do everything within their power to erase knowledge of their existance from the mortal world and that they are playing a long game.