Every word I say is true; this I promise you.
I think the pronoun 'this' is the direct object of the verb 'promise' and 'this' should be be placed after 'you', but it is placed at the beginning of the sentence.
Because this sentence is on a web page about English grammar at http://grammar.yourdictionary.com , I believe this is not grammatically wrong. Is this an exception? Then, how can it?