Clemethious Heterodyne is one of Agatha's ancestors, and the father of Bludtharst and Euphrosynia. 200 years before the start of the strip, he and his son led one of the family's most aggressive assaults on European civilization, an effort which was opposed by the Storm King Andronicus Valois. Very little is known about him; to date the only in-strip mention of his name comes in the historically-dubious that Klaus spins for Phil the Story-Teller. Klaus claims that Clemethious always smiled, even in his sleep.
It is the prologue to Agatha H. and the Clockwork Princess that offers what few details we've heard about him and his fight with Andronicus; for one, it is strongly implied that he and/or Bludtharst deliberately lured Andronicus into marrying Euphrosynia.