It implies that dark times and difficulty follow in his wake, that he always brings ill news, and its meaning is consistent with the opinion amongst Gandalf's enemies that he is a troublesome meddler in the affairs of others.
He always brings ill news. Dark times follow in his wake. Difficulty follow in his wake. His wake is this. This implies. He is a troublesome meddler in the affairs of others. Its meaning is consistent with the opinion amongst Gandalf's enemies this.