Watch Kikansha no Mahou wa Tokubetsu desu - Episode 6

Shadow Realms have ravaged the earth and decimated almost everyone in their way. The survival of humanity hinges upon the success of just six mages, including analytic fighter Desir Herrman. The remaining heroes make one final stand to end the bloodshed...but to no avail. Desir closes his eyes on what should be the end of his life—only to open them and see the beginning of his school days?! Desir has returned thirteen years back to the past, where he’s been given another chance. A chance to see the loved ones he lost, a chance to meet his fellow warriors again, and a chance to save the world…!

(Source: Yen Press)