It's about a hero who after defeating the evil King who transported him to the fantasy world is forcefully returned to his original world by his so called "friends" but he swore to himself that he will get back to the fantasy world to get answers and manages to do so after I think 20 years by using a girl from the evil kings bloodline (who (the kings bloodline) is hunted by his former friends (also for experiments)) as a portal and is now traveling with her.
Reloaded into the Other World
Summary and image taken from Anime-Planet:
When Emperor Rustanard of the Rusclan Empire summons the Otherworldly Savior from another world to fulfill a prophecy, he doesn’t expect to find 16-year-old Sihan Seong. The emperor casts him out, only to have Sihan return with a party of six warriors to defeat the so-called “Mad Emperor” and set the Rusclan people free. Just when victory is within Sihan’s grasp, he is betrayed by his teammates and sent back to his own world. 10 years later, Sihan returns to the Terranor Continent and finds an unlikely ally in Alita, the daughter of Rustanard…