The Eldrazi aren't exactly aliens. They are far far worse. Generally when Imagining aliens, people think of something strange sentient beings from another planet in our universe. Eldrazi are from outside the universe. They go from universe to universe, devouring them until nothing is left, then move on to the next one. While a few of them appear to be humanoid, they still tend to have very inhuman features. One of the usual variations is the scond arm branching off at the elbows.