Complex sentence: Tieflings were originally introduced as humans with demonic ancestry, whose lineage can be traced back to some degree or another to that of a fiend or demon within the "Dungeons & Dragons" universe.
Split into simpler sentences: Tieflings were originally introduced as humans with demonic ancestry. A Tiefling is a being whose lineage can be traced back to some degree or another to that of a fiend or demon within the "Dungeons & Dragons" universe.