I’ve been looking for a lore blurb or something and it’s driving me insane

From the Dark Heresy RPG supplement Disciples of the Dark Gods (2008) page 14:

///The following text was recovered from a sealed scroll case found clutched to the chest of a desiccated corpse on the hulk Lost Pilgrim, found drifting in the Hazeroth Abyss. The identity of the corpse could not be ascertained. ++ADM8567///

Sinophia Magna, Third Day, Eve of the Emperor's Ascension M41

Our negotiations go well, though I am anxious about the credit that my master is proposing to offer to these Sinophians. Their wealth is rotting out from beneath them and I doubt that they will be able to fulfil our terms for long. The weather is appalling and the locals worse.

Sinophia Magna, Fourth Day, Feast of Emperor's Ascension M41

In deference to the God Emperor, who watches over all, we conduct no business today. My master and I attended service at the cathedral in the morning. The cathedral is most peculiar. I like this place less and less. Though it is ridiculous, I feel I must report that during worship I found myself at one point staring at the glass of the stained windows. I could have sworn for a moment that I saw a dark orb, haloed with light, rise behind the glass. I blinked and it was gone. I did not mention it to my master. Maybe the damp in this place is breeding a fever in me.

Sinophia Magna, Fifth Day

Our business is not going well. At the negotiations this morning there was almost violence. A disagreement flared up so quickly that I am at a loss to recall what the point of contention was. Bloodshed was averted but our business is now voided. Our transit back to Scintilla is not due for five more days and I doubt we can travel earlier from this forsaken place. I am sure I have a rising fever. The light seems dimmer.

Sinophia Magna, Sixth Day

We were woken early in the twilight hours. There is trouble in the streets and we have had to confine ourselves to the manse. I heard shots fired several times throughout the day. I have developed a rash on my arm. My master seems tense. So am I.

Sinophia Magna, Seventh Day

The clouds cleared in the night though I wish they had not. I could not sleep and my fever rose in intensity. In the starlight, the city looked like the flesh of a rotting corpse. The fires from the riots still burned and I was sure I saw pale figures in the shadows on the street. The rash on my arm has spread; I think it looks like the claw marks of a bird.

Sinophia Magna, Eighth Day

When I woke, it was still dark. I thought that I was passing into a deep fever. People were in the streets. It was dark but people moved around as if conducting the normal business of the day. I went out and stopped a woman to ask why people were going about their business at night. She answered me but I did not hear what she said. She had the mark on her face, like a bird's claw mark pushed into soft clay. I awoke face down in the street. The city still burns.

Sinophia Magna, Ninth Day

The enforcers have declared a curfew and forbidden all travel. I hope my master can find a way of persuading them to let us leave. I did not sleep last night. When I close my eyes I see dead eyes looking back at me.

Sinophia Magna, Tenth Day

I must record what is happening. I was awake in the half-light of the morn and watched the sun rise. It rose, Emperor save me, but it rose black. At the moment of its rising, the city screamed. Things roam the streets and I see the claw mark on them all. They are changing. I do not know what they are becoming. I saw my master, his skin pale and stretching, eat the corpse of a servant. I have barred the door. The house is burning. I seal this record and someone must find it, someone must know. I dared to look in the mirror. I have the mark on my skin and I am not sure that it was my reflection looking back at me.

/// The above events described as taking place in Sinophia Magna correspond to no record of past events. ++ADM2190///

people turn to crow monsters or something

This is the only unique part of the story :)

I did a search and found the "Crow Legion" but that is on the Warhammer 40,000 Fanon wiki. What I read there didn't match what you were looking for.

It sounds like one 😅

Idk I don't recall ever even hearing of a similar story in 40k. Are you certain it's not from some other sci fi universe?

If you haven't read Dracula yet give it a try.