The Undertaker's real name is Mark Callaway. He made his WWE debut(then the WWF) in 1990 as "Cain the Undertaker." Before going to the WWF, he was in World Championship Wrestling and worked under the name "Mean" Mark Callous. He and Dan Spivey were the Skyscrapers.
Early in his career Undertaker wrestled under the names Texas Red, the Master of Pain, and the Punisher.
Mark Callaway was one of several talents that WCW failed to use properly. In addition to Callaway, WCW didn't see potential in Mick Foley and Steve Austin. WWE signed them and they all became major talents.
Most collectors of wrestling autographs look for signed photos, but the Undertaker has a lot of fans. A simple signature might bring $10.