Alright, to be honest, this needs some work. I read the first paragraph and my mental response was "wat". I got nothing out of it. I only understood after I read the other paragraphs, and it really helped that I already knew the story. The problem is that you have too many specific things that don't get explained. The 87th migration? Hel-Gorgath's minions? Ancients? Wtf are these? "Sharing names and ways with his underlings and allies on the other side of the Veil" - what does that even mean? He told them his name or something? The idea is fine - good and evil mortals are fighting, the All-father tried to help one side, the other Gods got mad and made him an outcast, then decided to make his half-god children fight for his spot - but the actual write-up is filled with details that don't add anything; they just make it fancy-sounding.
In contrast, the last two paragraphs aren't too bad. You use lots of unnecessary fancy words, but there are very few proper nouns; it is clear that the Progenitor had a bunch of children with mortals and the Gods sent messengers to get them to fight for the Progenitor's place. It is okay to have detail and lots of backstory, but you don't want to throw it at the reader immediately. Someone reading the manual wants to know how to play and wants a little bit of story so they know what is going on; if they want detail, it should be provided somewhere else, like in descriptions for each Demigod or in official fiction or something other than the introduction.
I would suggest something more like this:
"When the minions of darkness, led by Hel-Gorgath, were in rebellion, and the balance of light and darkness was under threat, the Gods convened a gathering. The Gods found that one of their own, the so-called All-Father, had intervened in the conflict by sharing names and ways of war with his underlings and allies in the mortal realm.
The Gods did not approve of this intervention. In punishment, they cast down the All-Father from the Pantheon and all of his servants and kin were destroyed.
The Gods therefore sought a successor. It was known of the All-Father that some of his many offspring, begat by mortal consorts, retained sufficient divinity to stand for induction to the Pantheon. But not all agreed, for some were offended by the obscene co-mingling of the essences of Gods and the crudeness of flesh. For this reason, a game was proposed for the offspring to prove themselves worthy.
So it was made to be that messengers were sent to the Dark and Light Places to summon the potential successors to the world of Rokkur. There, they would throw down their brothers and sisters in the arenas to win the favor of the Gods and ascend to their ranks."
I think that makes it a little easier to understand. I think the Hel-Gorgath part might not even be necessary, if he doesn't actually have anything to do with the game other than backstory. It would also be good to explain Rokkur:
"Rokkur was a world of eternal conflict. The mortal forces of Light and Darkness had fought for centuries with no progress made by either side. Thousands of majestic stone arenas were built on Rokkur by the opposing forces. These arenas were the sites of fierce combat and horrible slaughters. More and more arenas were built over time, using the old sites as foundations for newer places of battle, resulting in a massive layer of arenas around the planet. However, despite their utmost effort, neither side could defeat the other, and the stalemate seemed destined to last forever.
The children of the All-Father were sent to these majestic arenas to turn the tide of battle. Each of the offspring, essentially half-gods and know to the true Gods as Demigods, fought for either the force of Light or the force of Darkness. There were four Gods of Light and four of Darkness in the Pantheon, and the winner of the contest for the All-Father's place would give one side control. The Demigods assisted the waves of mortal warriors in an effort to destroy the great Citadel of their enemies, the headquarters for the forces at each arena, surrounded by defenses. The first to destroy the enemy Citadel would be the victor at that arena, and thus win favor with the Gods and move towards securing a place in the Pantheon."