I made a search and it seems it was never mentioned, so I guess it’s worth pointing out.
This should be without a doubt the most insane shootemup ever made, obviously Japanese and made for the “underground” scene by some group or guy.
It’s insane enough to get a page on TvTropes and I found it looking for “Mind Screws”:
Hellsinker is visually and musically very surreal, but the plot is even stranger than any of the gameplay would lead a player to believe. The full version of the game contains hidden poems, loads of cryptic Japanese text, Morse Code messages in the title screen, flashing screens in between levels and during bosses, distorted voices and all sorts of fun things to spend time decrypting. It’s pretty much House of Leaves: The Game.
Not sure I can link the actual game or if it has a commercial release. It can be found anyway…
There’s a nice guide here if one dares challenge it: http://shmups.system11.org/viewtopic.php?t=26334
The game is noteworthy not just for being the helliest of bullet hell, but because it also has some very complex mechanics going on for an almost unlimited depth. It’s like an allegory for life. There’s also a website all in japanese with LOADS of text, schemes and rules, hinting that there’s a rather huge mythology behind it as well…