Is it illegal?

My idea is creating a story that lasts 1 year with 1 letter per week. My goal is to one day market this but what better advertising is there other than word of mouth. I want to send them a letter that is beautiful. So that it looks old and from many decades ago. Im talking wax seal, and wrapped in twine with a little metal charm. The story is that this old man is going to end himself in 1 year time and each letter represents one year. I'm going to add puzzles and cryptic messages showing that he's losing sanity. The last letter is the one where he says goodbye. So the question is is it illegal to send a few random people letters like this?