It means that you have an unavoidable, negative fate in store for you. You are damned is very close, but that includes a bit of a sense of judgement. Damnation implies that, in some sense, the sentence to which they are damned is earned. However, one can be doomed to a fate through no fault of their own. If you speak of a ghost who is "damned to walk the night", you could use "doomed to walk the night" as a synonym.