"don't blink, don't look away or your dead!!" It's the weeping angel mod from the show doctor who!! It adds a new mob called the weeping angel unlike most mobs these are very rare and cannot be killed normally and can deal 3-5 hearts worth of damage and they can take out your torches and if your in complete darkness then your dead for more info watch this vid on youtube
http://www.youtube.com/watch?feature=player_detailpage&v=Xj446LPEy7E there is a link to the mod in the description of the vid