An Interesting Theory.

[Phillip]

His father wrote from beyond the grave, as he was just about to die when he wrote the letter.

Spoiler below!

At the end of Black Plague, Philip does the same thing. He writes to someone even though he's about to die. The cutscenes at the beginning of each game and the little blurbs shown when loading a level are all part of the letter he writes to Mr. X. That's why he mentions writing beyond the grave. He's talking about himself.

Ah, interesting.

Well I won't go throwing the idea out yet! I guess we'll just have to wait for it to release.