Hecate is a series of 3 monsters appearing in the Toei Yu-Gi-Oh! anime. Together they combine to form "Gorgon". They also exist as cards in the Bandai Carddass and sticker cards in the Bandai Sealdass. Each of them represents one of the aspects of the three-headed Greek goddess Hecate: the "crone" ("Red"), the "matron" ("Yellow") and the "maiden" (Violet") while "Gorgon" represents Hecate itself.
Risa Kageyama and her sisters stole the card from Yugi. When Yugi came to get it back, they took the Millennium Puzzle from him and forced him into a Duel. Without the puzzle, Dark Yugi was unable to come out.
When Yugi was about to lose, Seto Kaiba stepped to finish the Duel in place of him. He let the sisters put "Violet Hecate" into their Deck for the Duel. They Summoned the 3 "Hecate" cards and used them to Summon "Gorgon", which equally matched Kaiba's "Blue-Eyes White Dragon" in strength. Kaiba Summoned a second "Blue-Eyes White Dragon". Using both of them he was able to destroy "Gorgon" and defeat the sisters. He explained that he should be the one to defeat Yugi.