Future Demon Lord, Present Hero

In the year 2055, the human population was reduced to a meager 40%. In 2025 the world was besieged by a great calamity where the human race was challenged by another world to essentially a RPG-style death game. After almost twenty years the game was cleared by the strongest heroes left along with a man who eventually would become to be known as a demon lord. In a bid to change the future, stop the demon lord, and save more of humanity, the seven heroes banded together with an army to besiege the demon lord's castle to obtain an item known as the goddess tear. Two fledging followers of the heroes manage to get the tear and travel back in time; however, will their changes to the timeline be all that positive?