The Umbrex Plague, or Umbral Death, or Mutation-happy-Makuta-gone-crazy, all three have been used to describe the Plague. But whatever term is used, all agree that the Plague is extremely random, and does differing things to different beings. So far, only those of the Templum have shown the ability to resist its effects, due to their connection to the islands power. The Skakdi have shown a weakness to the Plague, and mutate very easily, but do not die as quickly. Xians mutate very little, but die rapidly. Matoran and Toa both show little mutation and die slower than either other race, but their minds are affected to the extremes.
The Plague can be caught by physical contact with a being who has the Plague for more than five seconds, or by inhaling it. The Plague appears as a black cloud when airborne, and the entire area around the old Brotherhood Fortress is covered in this cloud. Water it infects appears purple, and plants wither and turn black and decayed.
The physical change is the first thing that will be noticed. At first, the Plague will manifest in a purple, dark purple, and black flame with tinges or yellow and red that flashes up from some point on the body, and then leaves a great mark of the same color. Next, the change in color spreads across the entire body. Transformed areas then begin to bulge over time, becoming more muscular. Skakdi always manifest it first on the spine, and it then causes this part of the body to grow over the body, turning the being into an organic appearing monster. Xians begin coughing up their lifeblood, and armor begins falling off, as well as muscle decay, until they eventually die.
Matoran and Toa become larger, and develop oddities, such as hands that transform into huge claws, stinger tails, or small undeveloped wings. The oddest part is that, as the change spreads, it causes a half-and-half appearance on most. The Plague is a half curse, half blessing. On the negative side, elemental energy and hand-eye coordination is lost, and it becomes nearly impossible to use certain weapons. And everyone wants to kill you. However, on the positive side, it increases physical strength, gives those normally unable the ability to see in the dark, gives powerful assets like tail barbs and spikes, and grants shadow elemental energy. However, back on the negative side, you eventually go into states of violence, dementia, the urge to kill people, forget who you were, and eventually become a snarling monster with no control.