Artist Todd McFarlane created the classic comic book series about the undead superhero in the 1990s, but faced legal action from Neil Gaiman over the use of several characters, after he had been invited to work on an issue.
In 2002, a jury found Gaiman was owed payments for being a co-copyright holder for Spawn characters Medieval Spawn, Angela and Cogliostro.
Gaiman testified in June that he believed Dark Ages Spawn was a copy of Medieval Spawn, a character he created in the ninth release of the Spawn series in 1993.
Gaiman also claimed the angels known as Domina and Tiffany were copies of Angela, a character who also debuted in Spawn No. 9.
U.S. District Court Judge Barbara Crabb ruled in Gaiman's favour on Friday, declaring the medieval cast was in fact derived from characters created in his one-off issue.
An agreement for compensation has yet to be reached.