during the spell plague (you'll note an aweful lot of stupid bits of FR lore start with "during the spell plague") Asmodus became a god by consuming Azuth;s divine spark. his followers then did a ritual that claimed all teiflings in Faerun, no matter the herritage as "his infernal bloodline" absolutely stupid, the best I can figure is WOTC did this so they could eistablish a baseline of what teiflings looked like, but yeah... ABSOLUTELY stupid. FR is still recovering from the absolute rape that 4th edition did to the lore
