So far, the writing is rather plain. I like that the author of the letter (a lich, obviously) is actually rather condescending to his readers, it really gives him kind of an attitude. Other than that, the writing style isn't all that spectacular.

Consider the following five lenses for writing:
1. Character: you already have that. Your lich has a serious attitude problem, and is sarcastic and selfish.
2. Information: the trick is to convince your readers you were there--and that they are too. What is it like to be in the lich's shoes? What is it like, raising the dead? We are told it is a sin against nature, but why does he keep doing it if that is so?
3. Content: A chapter should be full of scenes, no fewer than perhaps two or three. Lots of things ought to happen in a single chapter, and the story should move at a brisk pace. This fic is kind of sloggy.
4. Variation: A good amount of it, but nothing that outstanding. A thesaurus would be good, but it helps to know how to weave together a sentence creatively, too.
5. Flow: The sentences occasionally don't mesh all that well. Each paragraph should be able to influence the next one, going from event to event naturally.

Take these five items into consideration, and you'll see some definite improvements. Thanks and take care.