The demon lord is an immortal being that can never truly be vanquished so every few years a hero must arise to seal him away. If one should fall before the task can be completed, the people must endure the demon lord's terror long for the next hero to come along and defeat him. This was the cycle that continued on and on in this world until one day a hero emerged that would time and time again seal away the threat. Unlike the previous heroes, this one was of the long lived elven race and was formiddable enough to walk away alive from these bouts with the demon lord. He guaranteed these victories to the point that the sealing of the demon lord became routine and civilization was able to flourish for many, many years. As an elf, he could live on long enough to keep the demon lord at bay for tens of thousands of years but tragedy struck him one day and he was left a broken man. This hero remains blissfully unaware of it but for him, time has stopped while the world continues to move forward.
Every few weeks, the hero's memory would spontaneously reset to a point prior to the tragedy that left him incapable of moving forward and every time it resets the point in his life he believes himself to be moves back further and further into the past. He keeps a journal with him to help him cope and adjust but whenever the time comes for him to do battle with the demon lord, his caretakers must take it away from him and hide the reality of his being so that he may carry out his mission without causing another major incident like he did in the past. After the demon lord is sealed, they would give him a new one and hide the true reason for the change by damaging the previous copy in some fashion. As long as he is alive and capable of defeating the demon lord, this hero will be made to fulfill his mission again and again and again until time forces the world to take a chance on another way.
This story revolves around the lives of the hero's caretakers and the pains they go to make sure the world stays at peace and to provide the hero with the comfort and mercies he deserves to make up for this cruel fate he's bound to. The two other long lived characters that take to the stage with the hero are his best friend who came from the same home he did and his own half-elf daughter who bears a painfully strong resemblance to her mother. At this point in time, the hero has just barely closed off a cycle and has gone through enough cycles to have forgotten his marriage to his wife. This along with the hero's dwindling powers is looking to make the next cycle the one everyone the most uncertain one as it becomes apparent that their reliance on him must stop.
I remember this getting a response or two. The daughter would normally pretend to be his wife during the cycles and everything boils down to whether or not his caretakers can bring themselves to finally let him die. It'd take place over several cycles and every refrain of a scene would only get more tragic for the leads as they all find it harder and harder to cope. Even the demon lord sorta takes pity on them and reminds them he always has the solution to their problem. The ending would have the hero share with his daughter the reason behind her name as he miraculously doesn't break down so badly in this final cycle that he's willing to forgive and accept the pain he's forgotten so that he can die.