My Familiar Is the Demon Sovereign

At Starfall Arcane Academy, scorned Verdant mage Elowen summons a dying black demon pup rather than a fabled beast. She names him Ren and defends him against all odds, only to uncover his true identity as Soren, the feared Demon Sovereign. Tied by a fateful pact, Elowen heals his broken arcane core with her life magic, and they redefine the Netherdeep together.