What sacrifice is Pyrrhus commanded to make in order to atone for his sin in the book, The Women of Troy: A Novel?

Pyrrhus is commanded to sacrifice his horse to atone for his sin, which leaves him overcome by distress. Ultimately, his love for the horse makes him recognize his own guilt, and he realizes that he must bear the punishment for his misdeeds, and must not let innocent Ebony suffer on his behalf.