To be honest, there isn't much to actually spoil because, well ... nothing is really revealed in the DLC. And at the risk of sounding like a broken record, that's what's so disappointing about it.
If you went into Witch Hunt expecting to have whatever questions you had at the end of Origins answered, with regards to Morrigan, the Dark Ritual, and the various whys behind her motivations, you'll finish Witch Hunt with those same questions, as well as a couple more to go along with it. IMO, a horrible way to finish off the final Origins DLC.