I will just silently judge you if you can't use any connection found between the two games.
As usual, a story where the devs haven't explained most of the things will always be speculation, but I am basing what I think the story is using whatever can be found in both the first and the second game.
So here goes.
Please do write your own theories, put them in spoiler however!
SpoilerShow
The purpose of Grimrock is the same as Nex.
You are a bunch of prisoners who got stranded and did all the trials of the island, encountered ancient technology along with the Goromorg religion(Might have originated from the Pyramid? along with the ending showcasing) and what would inevitably become the entity trapped in the cube. You got trained and hardened and weathered from fighting through the Island but there was no permanent way to destroy the Entity that would be housed in the Cube. Also known as the Undying One.
Your characters become the Guardians of the Island, and the Prison Grimrock is being built during this time using the plans found on Nex. Eventually you fight off the entity that would later be trapped in the Cube, and house it in the cube to stave off a portion of its power.
The treck on the Island of Nex forged your characters will and purpose, something they might have never had when they were initially imprisoned; and found a new calling other than what ever they did previously in their life. They would also have the Strength to found a country or at the very least persuade someone of high power due to the considerable strength you have gathered in Nex.
It would take hundreds of years to finish the plans for the prison in Grimrock (which you can find in the game), and Grimrock's purpose is to set off the otherworldy explosion that can finally banish the Undying One. Something only the Guardians were strong enough to do.
The plans for Grimrock is a test of strength, not quite similar to the ones who trecked Nex. Since the Guardians(You) would not be able to live for as long as it would take to build everything inside Grimrock, they made a Royal bloodline along the way or had it swear a fealty of protection (And maybe had the Architects change a bit of the original plans). Said line would always banish prisoners to the top of the mountain, and if their will and purpose was strong enough; just like the Guardians - to be free from it all and forge a new path; they would be as strong, if not stronger to defeat the Undying One and finally detonate the mountain in order to banish it.
In order to detonate the mountain, the Undying One would have to be defeated in order to set off the magical reaction required to banish it.
The Undying One only believes the prison to only be there in order to house him.
The Goromorgs who worship The Undying One knows of its purpose and wants to stop the Prisoners.
You are a bunch of prisoners who got stranded and did all the trials of the island, encountered ancient technology along with the Goromorg religion(Might have originated from the Pyramid? along with the ending showcasing) and what would inevitably become the entity trapped in the cube. You got trained and hardened and weathered from fighting through the Island but there was no permanent way to destroy the Entity that would be housed in the Cube. Also known as the Undying One.
Your characters become the Guardians of the Island, and the Prison Grimrock is being built during this time using the plans found on Nex. Eventually you fight off the entity that would later be trapped in the Cube, and house it in the cube to stave off a portion of its power.
The treck on the Island of Nex forged your characters will and purpose, something they might have never had when they were initially imprisoned; and found a new calling other than what ever they did previously in their life. They would also have the Strength to found a country or at the very least persuade someone of high power due to the considerable strength you have gathered in Nex.
It would take hundreds of years to finish the plans for the prison in Grimrock (which you can find in the game), and Grimrock's purpose is to set off the otherworldy explosion that can finally banish the Undying One. Something only the Guardians were strong enough to do.
The plans for Grimrock is a test of strength, not quite similar to the ones who trecked Nex. Since the Guardians(You) would not be able to live for as long as it would take to build everything inside Grimrock, they made a Royal bloodline along the way or had it swear a fealty of protection (And maybe had the Architects change a bit of the original plans). Said line would always banish prisoners to the top of the mountain, and if their will and purpose was strong enough; just like the Guardians - to be free from it all and forge a new path; they would be as strong, if not stronger to defeat the Undying One and finally detonate the mountain in order to banish it.
In order to detonate the mountain, the Undying One would have to be defeated in order to set off the magical reaction required to banish it.
The Undying One only believes the prison to only be there in order to house him.
The Goromorgs who worship The Undying One knows of its purpose and wants to stop the Prisoners.
SpoilerShow
The Island Master knew of the events that would transpire and had set up the island for the purpose of fighting off the Undying One.
