Darkbringer is an evil, godlike being who plots the corruption and theft of souls. When he corrupts parents, they may have a tiefling child. Darkbringer takes that child at a young age and indoctrinates them to be a loyal servant of evil.
There is no place in Lasthold society for these tieflings, even if somehow a tiefling finds a conscience and breaks from evil. Generally, humans capture tieflings and execute them. Dwarves kill them on sight. Elves like to torture them and interrogate them before they die.