For a few years in the early to mid-'60s, Dick Radatz was as dominating a relief pitcher as baseball has ever seen. He still holds the record for strikeouts in a season by a reliever with 181 in '64.
http://sports.espn.go.com/mlb/news/story?id=2014955
http://sports.espn.go.com/mlb/news/story?id=2014955