Sangrazul
Sangranzuls are an anthropomorphic race which have the ability to find mana lines better even than planeswalkers. As such, they serve them often. Currently there is only one known Sangranzul - Malvos, who served the planeswalker Tempè.
Sangranzuls have great longevity. They do, however, need to "eat" mana from time to time to survive. Death by starvation, though, is a long process. After at least five hundreds year, starving Sangranzul start dying. It's very painful and months of concious horror can pass until the immovable, drying body releases the tormented soul.