Fan CastingThe X-Men III

Description of Mister Sinister (a.k.a. the Professor's first secret):

For years, Xavier has known about the existence of Mister Sinister, a scientist born in the 19th century who has been experimenting on mutantkind for generations. Nathan Essex was only a clone of the real Sinister, who has also been involved in the pasts of many X-Men. Sinister threatened to destroy the Xavier Institute and Xavier's students, so Xavier agreed to look the other way to protect his students. He also recruited Nightcrawler as a secret spy to keep tabs on Sinister. Xavier himself admits his knowledge of Sinister’s existence, fracturing his relationship with his students.