The ultimate fear of mankind is a mutant so powerful and so dangerous it can never be stopped, it can never be controlled. The X-Men faced threats both powerful and pedestrian for years before confronting the menace of Proteus.
Known first as Mutant X, Kevin MacTaggert was a nightmare: an emotionally abused, psychotically unstable mutant who saw other living things as toys, and had the raw power to make his vision a reality. His vampiric need for fresh host bodies to contain his energy form only made the danger of Proteus that much more chilling. "X-Men Don't Kill", a rule often cited by team leaders Cyclops and Storm, was challenged and ultimately broken by the unrelenting danger posed by Proteus.
Even dead, Proteus wasn't gone forever. Often scattered and dispersed, but never truly destroyed, Kevin MacTaggert returned time and again. Occasionally flirting with a sympathetic or even heroic change of heart, Proteus always reverts to his villainous origins. Proteus is a threat to everything that lives, and will never truly die. Few villains justify the continued existence and vigilance of the X-Men as well as he does.