Dark manuscript is a script found in the Arceuus House Library in Great Kourend. Horphis may ask the player to find the script for them, after which the player is rewarded with 5% Arceuus House favour for each script handed in. The location of the script varies by world. Horphis will not talk to players until they have at least 20% Arceuus House favour.
To get a hint to find the location of a manuscript you can ask Biblia on the top floor of the library.
Players cannot read the manuscript as it is too complex for the player to decipher the symbols.
Players can gather multiple manuscripts at a time and hand them in in bulk (4 maximum) allowing for faster favour gain. This is done by returning to Biblia on the top floor after each manuscript is found, and asking for the location of any dark manuscripts again.
It seems that every time the locations of the manuscripts are reselected, 2 random locations are chosen for each rooms, and the manuscript will always appear in one or the other location.
It can be useful to note the locations for faster gathering.