Diin Adarin - Pyromagus (Demonblood Magus)

This is an idea that sprung from reading my friend Adkit's novel and seeing a reference to a wizard whose fire spells could burn fire-immune creatures like efreeti and red dragons.

Diin D'Adarin is one of the most powerful and best known spellcasters in Vane. He earned the title of Pyromagus for demonstrating such an utter mastery of fire magic that he can burn a fire elemental to death. But it is not for his power that he is most well-known. Rather, it is his habit of never wearing anything but a smirk. Fortunately for the inhabitants of Vane, Diin is a handsome man of muscular build and wears his hair long, which occasionally grants some reprieve from his unclad form. [I imagine him looking like Suiryu from One Punch Man, though a bit older.]

As appropriate for a mage of his power, Diin is a member of the Magister Council and has a habit of dropping in whenever anything interesting or important is being discussed, even when steps are taken to keep such discussions secret. Other than his involvement with the Council, he usually disappears for long stretches of time to practice his spells in the Westlands to avoid damaging the city.

Though born with prodigious sorcerous talent, Diin sought any means to increase his power further. Prior to becoming the Pyromagus, he succeeded by transfusing himself with the blood of a demon he captured in the Westlands. The blood infused his magic with traces of hellfire, greatly increasing its heat and allowing it to penetrate innate fire immunity. After the procedure, however, he became emotionally unhinged, quick to anger and hungry for the sight of vengeful flames burning the world to ash. He wanted to see the masses tremble at his feet before being consumed by his unstoppable fire. Just enough humanity remained for him to escape to the Westlands and burn himself out. Since then, he has adopted a confident, carefree attitude without inhibitions. He does what he wants, when he wants, where he wants; and when he feels the urge to burn, he travels back to the Westlands to release the roiling hellfire.

Archmagister Ghaleon is aware of Diin's demon blood and has arranged contingencies should the Pyromagus ever lose control in Vane. But he views Diin as a valuable asset and an exceptional mage and does not see any issue with his past pursuit of power.

In 5e, the most straightforward build for Diin would be a Sorcerer with the Pyromancer origin. It allows fire spells to affect creatures with fire resistance as though they were not resistant and, later, even fire immune creatures are treated as only resistant. The other option is the Elemental Adept feat, which overcomes fire resistance just like the pyromancer ability and allows Diin to be an actual wizard with Evocation abilities (though it means overcoming fire immunity is a hand-wave ability).