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Originally Posted by Tellerve Black Harbinger lasts until the end of the encounter as it states in description. |
Which, while being a true statement is neither useful, nor the answer to the question you asked.
Anyway, wildshape is permanent as per durations in PHB. Black Harbinger lets you use wildshape to change into an actually useful form. Nothing says you lose wildshape at the end of the encounter when you use Black Harbinger, so you remain in raven form until you intentionally leave it.
Interestingly enough, as wildshape is written you're unable to LEAVE raven form until you use Black Harbinger again. So if the encounter ends and you're still a raven, you're SOL until you can get the utility back.
RAW are wonderful!
You'll want to houserule it to either have it say you leave raven form when the power ends, or allow wildshape to work from any form you happen to be in (not just human and beast) into human and beast. At least until PHB2 gets errat... er... updated.