House Costayne are a noble house from The Reach and are vassals of House Hightower. Their seat is a castle called Three Towers. Their sigil is quartered, with black roses on gold silver chalices on black. The current head of the house is Lord Tommen Costayne.
Lord Owen Costayne sided against his liege lord Ormund Hightower and declared for Queen Rhaenyra Targaryen during the Dance of the Dragons. Owen was mortally wounded, however, by Ser Jon Roxton during the Battle on the Honeywine. He died of his wounds later on. It is unknown whether the Costaynes continued to support Rhaenyra after Owen's death.
Ser Leo Costayne served as Lord Admiral of the Reach and was hired by Johanna Westerling to get revenge on Dalton Greyjoy, who had burned the Lannister fleet and sacked Lannisport. Leo made some successes initially, surprising the Greyjoy fleet, but is eventually slain. The Costayne forces return to Lannisport with some highborn captives
The Costaynes sided with King Daemon Blackfyre during the First Blackfyre Rebellion, fighting with their king during the Battle of the Redgrass Field, where the Costayne forces fought on the left. The daughter of Lord Ambrose Butterwell married the Lord of Three Towers at the time, and the man took part in the wedding festivities of his goodfather. He participated in the Whitewalls Tourney but was unhorsed by Ser Glendon Flowers.