Majutsu no Hate wo Motomeru Daimajutsushi: Madou wo Kiwameta Ore ga Sanbyakunengo no Gijutsu Kakushin wo Kitai shite Tensei shitara, Kanashiku Naruhodo Taika shiteita......

After a cruel and calculated reign, Kai Lekius, the Sanguinary King, brought stability and peace to his once fragmented land. Thus began his deathlike slumber as he awaited the rise of magic, leaving the rest in the capable hands of his brother. Three hundred years later, he wakes as a True Blood, an immortal vampire of immense power. However, even after all that time, magic hasn't progressed at all! A nobility system, which he has no recollection of establishing, tyrannizes the weak and weary of the new world. Though it displeases him greatly, Kai must once again postpone his studies and save his people once and for all!

(Source: J-Novel Club)