Chronicles of Eorde
Humans aren’t the only descendants of the highmen. Though many would like to think otherwise, the renegades were not completely destroyed by the dragon’s rage.
Those traitorous highmen that survived broke away from the drow, who fled underground. The traitors fled in a different manner, the warlocks among them opening a portal to the dark realm of their patron, the Primordial Dark. Over time, their forms changed to better suit their living conditions.
After many years, their descendants found a way to return to Eorde, determined to right the perceived wrongs their ancestors had suffered. They called themselves tieflings, a bastardized word of the ancient highmen language, roughly translating to “children of the dark.”
The diabolic visage of the tieflings is due to their exposure to the dark magics of the far realm and the twisted experiments of their ancestors. Though some can pass for human, most are quite obviously touched by the foul energies of the outer planes.