When in ancient days the black-headed retreated from the world, when, after days of war, the carrion birds circled, when graves were filled with the caskets, when the lone orphan plowed the barley--
--now at that time were the primordial patterns laid to rest, their masters banished. But just as the seeds of the next harvest are sown, so too did kings plot and plan for the next battle, did they scheme and make secret dealings.
Far to the east, where the sea and the mountains meet, is a cave dug out by the crashing waves. Here the kings conspired and set aside the greatest of primordial fires, Dingergisbar.
With its name as the key, it lays in wait, until that day when next masters of patterns again reap the harvest.
About the Note
This is a light-tan note written in Sudran.
Dingergisbar is apparently the Sudran word for the FlameThrower, evidenced by this being the passcode to access the weapon and the text "its name as the key". The lines referring to a "cave dug out" where kings "set aside the greatest of primordial fires" must be referring to the watery path leading to the Flamethrower.
It's notable, however, that the Note says this cave is "far to the east", while the room containing the flamethrower is on the far west side of the map. This could imply that the Sudran civilization was founded on another area of Sudra, which is inaccessible in-game.
"Far to the east, where the sea and the mountains meet" may also be a reference to an area that in the end never got created. From the Axiom Verge AMA on Reddit: "There was going to be a small outdoor ocean-themed area - basically three or four rooms worth. It was a lot of assets to make for something that didn't add very much. If you have the soundtrack, the bonus track would likely have been used for that area." - Tom Happ
The "primordial patterns" referred here may be a reference to the Primordial Code.
This is Note 06.
Mar-Uru, under the room to the right of the room above the Sentinel. Corrupt the Purple Sentry and navigate it up and right before entering the door so it appears in the box with the switch to lower the pink beams. Use the Red Coat to pass through the floor. Here's a video.