She's a Passerby, But Can See the Protagonist's Halo - Chapter 23

A supreme expert in the Immortal World had died, and a strand of his soul returned to its original body on Earth. Tang Xiu discovered with amazement that ten thousand years passed in the Immortal World, yet only a single year passed on Earth.

Ruthlessly betrayed by his lover and trusted friends, Tang Xiu found that his soul went back to his previous body on Earth, and, despite he spent 10,000 years in the Immortal World, just a year has passed on Earth and many things have changed. Having an unsurpassed knowledge, Tang Xiu will protect his loved ones and go back to the Immortal World to take his revenge.

- Description from Novelupdates

He became a teacher at the best magic academy in the empire.
….For being mistaken for someone else.

After transmigrating, Chen Yu found himself in a highly militarized world. But the “will of the world” had concluded that he was an intruder and put a curse on him.

At the end of the story, I found myself in the body of a hidden villain, betraying the heroines. Since I’m not particularly concerned about the end of the world, I’m calmly waiting for the finale… “You’re the only one who truly appreciated my painting of the boa.” “When I make plans to meet you at four, my heart starts racing at three... is this what they call being tamed?” For some reason, the heroines are starting to take an interest in me.