From History Channel's Vikings, s3e2, "The Wanderer". See full episode at www.history.com/shows/vikings. Great show, check it out if you haven't already.
In the war for the crown of Mercia, Ragnar's forces, fighting for Princess Kwenthrith, cross the river to attack Kwenthrith's brother Prince Burgred. Seeing that the approaching ships are decorated with severed heads, Burgred's forces flee in terror.
Re-uploaded because the previous version had unsynced video/audio and overall poor quality.