This is book one of the Rift Magus Reborn series. You can find book two

I was the Rift Magus—master of space and time, protector of worlds. For decades, I defended my realm with unmatched power. But when an unspeakable creature threatened everything I held dear, I was forced to create a rift to nowhere—a trap designed to seal it away for all time.

It worked, but the creature was not without power itself. It warped my magic, twisting the rift I made to take me back home, and I woke instead in the frail body of a young noble, Lucian Alastair Eldraine. Beaten, ransomed, and on the brink of death, Lucian's spark was barely a flicker, his power nothing to speak of.

But I am not Lucian. I am the Rift Magus, and though my strength is diminished, my knowledge remains. Now, I must rebuild my power from the ground up, reclaim the magic that was lost, and rise once more before the darkness that haunts me finds its way into this world.

I’ve reached the pinnacle of power once before.

I can do it again.