The offspring of fiend and human by intercourse or ritual. These can happen during a war, or a cult summons a fiend to do the deed, and many more ways.
Tieflings are hated by generally everyone. From an early age, they are regularly visited by fiends in their dreams and promise of power and glory and anything else that would interest that particular tiefling. Because of this, it’s inevitable that a tiefling joins with the fiends and attempts to do their dirty work in the world of Amalna.