|The following is based on Fallout 3 cut content and has not been confirmed by canon sources.|
Obadiah's letter to Marcella is a paper note which was cut from the final version of the Fallout 3 add-on Point Lookout. It can only be obtained with console commands and is apparently a leftover of the original version of The Dark Heart of Blackhall.
I'm not imagining it; they're getting closer with each passing night. The dogs are gone - or taken for sacrifice, more like. I don't like to rely on superstition to keep them at bay, but there's no way I can fight them off if they take a mind to attack my home.
I don't know who else to turn to, Marcella. I'm skeptical, but I think we may be able to trust this outsider to help.