The one I:m talking about is LTDs mod. It used blood as a sort of xp/ stamina bar. You could bite npcs if you sneak up on them to get more blood. Running out of it would cause you to become weak. Once you level up your blood amount by drinking more you can choose from various powers. Some of those included teleportation, ethereal form, turning into a wolf or a swarm of bats. Aslo you could show/hide fangs to go incognito and you had a separate bounty as a vamprire, plus guards attack on site and dispostion is lowered.