Several large bundles of hand-written papers — sheet music, choreographic instructions, color diagrams, costume designs, architectural drawings, early-20th century maps of the Himalayas, mechanical drawings for unusual equipment, etc. Mostly in Russian, although some few in English. Together these comprise a nearly complete collection of instructions for Alexander Scriabin's Mysterium.1
For those of you who slept through music history, shame on you. See the following.
For those of you too lazy to click a link, Scriabin was an early 20th-century Russian composer and Theosophist. His final, unfinished work, Mysterium, would have been performed in India over seven days to destroy and recreate the world and the human race. Really. I don't know why Ken Hite never did a Supressed Transmission on A.S..
Any music historian worth his or her salt will want to get this set into general knowledge. From there, it's only a matter of time until some symphony tries to recreate it. Probably a Russian group, but merciful Keepers might recall AUM's orchestra and just let cultists try to play the whole thing.
ObDG: Not quite ARCHINT, but close. Reconstructions of this piece exist, but generally don't recreate the musical effects—much less the Mythos effects. With a big bundle of easily-divided material, one could have the classic EMERALD HAMMER-style paper chase, but with some unusual players: music historians and musicians plus the usual cultist, Delta Green, SV-8, Karotechia, Fate suspects.
Delta Green must take more care than usual
You can silence a cultist, or even a bronson without too much problem. Let's face it—if you've been too busy keeping secrets to talk to your parents, they're not going to miss you if some strung-out overstressed near-nutso Fed puts a bullet through your brain or too many Percodans in your food. Killing a famous musician will attract LOTS of attention, including from well-heeled patrons and parents.
Taking this from the opposite direction
Perhaps you could have a campaign where the musicians or art historians start by finding hints of lost Scriabin manuscripts. He remains more popular in Russia than elsewhere, so you might have a foreign angle to play. How does Joe Average Cellist and Jane Q. Costume-Designer respond to the creepy Aryans asking where they found these papers? And when the elderly Russians show up? And when the creepy NYC rocker starts asking? And when the US Marshals ask you to help them find the art thieves, or when the FBI says you need to play along with nice man from South America, do you get any idea you're in over your head?
Gotta Catch 'Em All!
This easily divided and reconstructed artwork could serve as a steady type of clue for a Castaigne Collection campaign. Interpol tracks down an art thief in Italy with some unusual Russian papers, which the embassy would very much like back (cultural treasure, let's not have a repeat of the Amber Room incident), but the FBI wants to talk to the same thief about his buyer in New York, and so on.
Tatiana Schloezer was an avatar of Nyarlathotep. These papers exist as a reality shard, evidence from a previous version of reality—where Scriabin succeeded in recreating the world. If you have the whole papers, could you repeat that act? Could you undo what had happened? Could you control the event for a particular end? Could you do it over and over?2,3
Scriabin's event, Version #1
Scriabin's event, Version #2
…was designed to recreate humanity as something nobler. What did this mean? A signal to the Yithians for mass mind transfer? If not them, perhaps Lloiogor, or some other quasi-material thingie. Perhaps it was designed to kill off the base impulses of humanity and set free the higher, better selves. That might leave some nasty and vicious successes4, or some nasty and vicious replacements5, or some nasty and vicious failures6.
Be seeing you…