(Short lore / story ahead, skip to the end if you want my thoughts and info)
The building had always been there, a creation from the ancient builders, created with magic and skill. When they disappeared, no villager had any knowledge on what it was or how to maintain it, only that the two orbs of wealth were to never be touched or stolen.
Over time, the building slowly fell into ruin, the bricks becoming cracked and overrun with vines, the chains almost invisible from the bushes coating them. At one point, a pillar split in two, causing the village to fall into a panic, but a tiny amount magic, left over from the ancient builders, kept it upright. The obsidian cracked and shattered, and it is rumoured that when it cracks, its wails can be heard echoing throughout the fields.
Many theory's where made on the building, and the purpose it served. Great librarians and clerics, the wise men of the village, came to the conclusion it was built to act as a wormhole, a way of passing through to a different realm, but the magic of it slowly faded with no one to upkeep it. They were almost right, the building was a form of portal, but it was not created to allow easy movement between realms.
It was created as a prison.
It was a doorway to a prison of the highest security, an entire dimension, created by the ancient builders to hold an enemy of unimaginable power, one so destructive it scared even them.
What the villagers didn't know was that the building was a door. A door that was locked tightly, but those locks where slowly rusting, with this great enemy waiting patiently in the hellish landscape within.
The world had forgotten it, but it remembered. It knew what it was called, what its purpose was. It was a creature of pure evil, of destruction and plague.
(Decided to try and right some story / lore for this build out of boredom, first time trying! It was created for @otherside-wanderer buildtober, with the prompt Ruin which made me think of ruined portals. I'm half happy with how it turned out, especially the pillars, but sadly the max portal size in Minecraft is 23x23, so the pentagram on it looks a little goofy. I might work on it some more and post the update later!)
Feedback / Comments on both the story and build appreciated!