Humanity struggled to beat the Tower of Destiny. Failure, they were told, would result in eternal slavery.
The first time the Black Reaper beat the Tower, he was given a choice: return to the past with his memories intact or buy freedom. Unfortunately, he only had enough points to buy the first one.
The second time he beat the Tower, the same happened. So did the third, tenth, fiftieth, all the way to the ninety-eighth.
As desperation hit, he searched for alternative ways of leaving. He discovered the Fate Keys, but the Last Key always eluded him.
It's now his 99th run, the last run the Tower would allow him. And it's time to face the harsh truth: it was impossible to find the Last Key or amass enough points to buy freedom
Humanity has already lost.
So he started over with the objective of making his final run the most memorable of them all. He would unite all races in the Tower into one final push. When humanity ultimately became slaves, they would at least be able to look back at their past with pride.
It wouldn't be easy. It would require him to do some questionable things at times. But he would do it.
He would unite humankind under his rule, he would unite the entire Tower under his banner, and he would triumph.
He dubbed himself the Merciful One.
He abused his knowledge of the system to grow into an unstoppable monster.
He shaped the Tower under his will.