The demon lord has been enslaving your people to dig in the obsidian mines for who knows how long. Something about gaining one soul for all eternity, and then multiplying it somehow.
One day you discovered quite by accident that the laws of hell allow your people to have a representative.
You voiced to the demon that was whipping you that you wished to become your people's representative.
He just laughed and continued whipping you.
However, a few days later, a dark wagon showed up in the mines. Those usually came full of fresh prisoners, who would be ravaged and then put to work.
It came empty.
The dark knights within singled you out and brought you into the wagon. You were took to a torture chamber.

That night, your eyes were plucked out.

You were taught to read with your hands. Ancient clay tablets. Thousands upon thousands of them. You were whipped every time you recited them wrong.

Somehow, you stopped aging. Took centuries of torture, but eventually you memorize all of Hell's Laws and Customs.

Then you were taken to the presence of the Demon Lord.

"Speak now. What is it that your people crave?"

"We crave to be Free!"

Now was the turn of the demon lord to laugh. It started as a minor chuckle. And then it became a booming laughter. And then all the demons started laughing as well.

"Very well. You owe me two souls for everyone I am freeing. The souls cannot be taken, they must be willing sacrifices. I will simply employ them in the same fashion as I've employed your people. If you cannot repay within a thousand years, I will reclaim all your people as slaves again, and also the entire plane in which I will release you. You will have to serve me for ten thousand years without complaint before you can strike a new bargain."

All his terms were in accordance to Hell's Laws and Customs. You couldn't help but agree.

He formed an island in the middle of the ocean in one of the myriad human planes. He then opened a chasm portal all the way from Hell into that island, somehow.

"Go forth and conquer this world for yourselves. Spread and remember to sacrifice the required amount. Once you pay your debt, we'll trade sacrifices for magical boons."

According to Hell's Law, a Representative must know all of Hell's Law. Merely by knowing it, he becomes Immortal. You don't understand how it works, but this is the sort of magical forbidden knowledge that changes your very being. There can be only one Representative, however. Being immortal doesn't mean you can't be destroyed. If you are destroyed, someone else Could take your place, but the spot could also be vacant for many millennia.

You have around Two Million People, unskilled, starving, who have known nothing all their lives but toiling in the obsidian mines. The island is huge, but the grimm reality sets in. Many will starve to death before you can figure out a way to eke out a living. You owe the Demon Lord Two Million Souls, give or take.