One of the beings on this world - they said he was mad - claimed to have found evidence that theirs was not the first intelligence which inhabited their home planet. There were others, from long ago, beings whose intelligence so far eclipsed their own that the young race was like mere vermin; others, who made space and time itself their playthings, and who were still here, hidden somewhere deep under the equatorial mountains. This scribe wrote furiously all he could about them, somewhat in resignation, somewhat as a sick joke. For he wrote that when these beings, these Great Old Ones, awoke, they would bring an age of unendurable, unending torment. The best anyone could hope for was to be among those eaten first.
The Mad Scribe said it had found their machines. Some reported that the Scribe was torn apart in plain view of others by invisible forces. Later this was regarded as naive legend.
Until the stars aligned, and the Great Old Ones awoke.
It began as the same nightmare experienced by artists and monks all around the world, taunting them with the inevitability of Their return, and the pointlessness of suicide. Then it was a team of explorers whose curiosity triggered it, who saw the monstrosity erupt from under those very equatorial mountains. It unfolded with an impossible symmetry - the shapes itched, because they could be seen but not understood. The explorers stood, in awe and nausea. And then It heaved free of the rubble and rose into the sky. And looking up at It, simultaneously many of the explorers went mad.
"Hello?" the thing said. Its voice jellied their very brains. "My name is Jake. I'm the first one awake." The tiny creatures in front of him spasmed with psychic pain.
"A straight line!" one of the explorers cried. "Euclidean geometry! Angles which are either acute or obtuse! O the horror!"
"Hmmm...you know, It's just a line," Jake said. "I didn't mean to uh -"
"O look at it!" they screamed. "O how grateful am I for the poverty of language, for its hideousness cannot be expressed by mortals!"
"Come on, I have acne," Jake whined.
"Please, eat us first! Now that we know such a thing as you can exist, please bless us with oblivion!"
"Look, this is not good for my self-esteem," Jake said. "I really don't think it's that bad."
"Oh look at it, a color from beyond space, it is the fabric of madness itself!"
"Maroon? Merino wool?"
"GAAAAHHHH!" and with that, the whole planet heaved a gasp of soul-destroying agony and expired.
"Well that's sad," Jake said.
If you think that was cheesy, the other way I thought about doing this was to re-write Flatland with the three-dimensional shapes as the Great Old Ones. And you know what mister? If I hear any more groaning from the peanut gallery I just might do it too.