The elbow is one of the most lethal weapons of a fighter and if used correctly can end a fight very quickly. Nathan Carnage Corbett, 11-time Muay Thai World Title holder is well known for his devastating elbow strikes. The video below is a good compilation of many of his knockout elbow strikes.
The next video is a demonstration by Nathan of how to deliver the elbow strike:
The next video is also a good demonstration of the 4 elbow techniques use in Muay Thai.