Story idea for manga/anime

The story takes place in one of those traditional ecchi harem magic stories and school life.

The main guy and several girls around him, but he already knows the girls.

The guy has weak magic (all others girls have way more magic power) and he is kinda useless, but all girls like him, and he likes them back (ik, it's like any other story, but calm down, it will get more interesting).

It starts with the main guy trying to save one of the harem girls (the main girl) from some kind of danger (small one), and find himself in an ecchi situation with her, but all the others girls see that and confuse it thinking he did that for no reason (without knowing that he tried to save the main girl, that I'll name her numb 1 girl).

Due to that, all the girls want to punish him, and use a way to do it that no one ever did before, locking him in some sort of container made to store goods etc, but not a human-like. The container is like a gem the size of a hand.

The girls plan to trap him there for 3 days, but by mistake, they take the wrong container, one that have a ultra age function (like, one day in that container will age anything inside for 1000 years, or 1 millennium. It's a rare gem that was not supposed to be there)

So, the girls make a ritual and trap him there as a way to punish him, but it was supposed to be for 3 days, but due to the gem hiper age function, 3 days are like 3000 years.

After 3 days, the girls release him from the cage, but the guy is not a human anymore, is a dark thing, necromancer/reaper like, only with the bones and a dark aura around him (to make it ez let's call him The Reaper).

In that world, the girls from that school are supposed to fight things like him, monsters etc.

Upon released from his cage, The Reaper jumps from the gem and touches the main girl and another one with the hands, draining all magic power and then, all others start to attack him, but fail to kill him. In one moment, the main girl notices that The Reaper still has a necklace that she gave to him, and alert the others that he is actually the main guy, and notices the mistake about the gem, not 3 days but 3000 years has passed by for him.

Inside the gem he was unable to contact anyone and neither die, the first few days he was just mad, but then months passed by (this inside the gem, showing the main guy) and he started to panic... years passes and he shout in rage and hate "-Why?! why did u do this to me?! It was a mistake, please let me die! I can't take this anymore!".

He slowly loses his sanity and humanity while the time pass by for him. By the time he is released, he doesn't remember who he is anymore.

But he doesn't fight back the girls. He only takes all attacks till they stop and notice it was the main guy and not a monster.

The Reaper starts to remember he was once a human, and asks "-Plz, have mercy on me and end my torment!".

But, due to the fact that all girls love him, they put him to sleep, rejecting his death request.

Since that (the fact a human was trapped inside a gem and aged like that) never happened before, the girls don't know what to do to save him, but since he (Reaper) drained the power of 2 girls, he gets back to the human form at the night, and regains most of his memories, but also he gains a huge power, never seen before.

In his mind (Reaper), he still loves all girls, but has a huge hate as well, due to the fact he was trapped and suffered so much for an unfair reason.

The story will keep going in a way that, he fights monsters with the girls, but also tries to find a way to die (since he become so powerful that he can't suicide) while the girls try to save him.