Time Slip of the Fist King

In a world where humans enslave other races, the archmage 'Laefenhart' is treated as the Demon King by the humans for his sins of helping the other races. A group of Heroes came to subdue the Demon King! 'Laefenhart' succeeds in causing great damage to the Hero party, but ends up being in danger of being killed by the Fist King, ‘Tesron'... "Space-Time Return Spell." The magic spell cast as a last resort miraculously succeeds and sends 'Laefenhart' to the past. However... I became one of the Heroes, the Fist King, 'Tesron', who had the strongest body on the continent?! The archmage 'Laefenhart', with his newly acquired physical strength, changes the world with new powers!