Otherwise known as Tieflings and Aasimar (Mongrels/Half-men)
There are no celestials or fiends in this world (Note: there are things that are mechanically identical, but no outsider is friendly. And only elementals are even comprehensible to mortals). Tieflings and Aasimars descended from familial lines that have been altered by magical exposure. No one has quite figured out what exact triggers the change in one’s blood, to make their children spellbloods. It’s known that a familial line can be specifically cursed with it, but it can also happen purely by chance. As for what triggers the change so that one line breeds Tieflings and another Aasimar, that is also a mystery. But one that many academics and wizards are interested in uncovering.
Appearances vary widely from one spellblood to another. But the general trend is that Tieflings are inhumanly, disturbingly beautiful, while Tieflings are simply monstrous.
The genesis of spellbloods is seemingly random, and in some bloodlines the curse has faded; seven generations of magic-twisted monsters and then back to normal. Due to this, there is not really an overarching culture for either species. The only reason they are known to be a species is the recurring characteristics such as all Aasimar being able to summon light and Tieflings darkness. Culture is restricted and unique to each family, and each group. All spellbloods are outcasts though; all are cursed, or considered to be.
Spellbloods tend to have very extreme feelings towards magic. Some hate it with every bitter fiber of their hearts, while others find it endlessly fascinating; compelling despite the harm it’s done them. And also factoring into opinions is the fact that all spellbloods are capable of some magic innately, with many possessing the full might of sorcerers. Even those who lack in that raw power still find that magic comes naturally to them. A basic fact of spellblood nature is that it is the one thing while they can hate or love magic, they can never ignore it. It’s a part of them, from birth to death.