To find and assassinate Deineira in The Fall of Deianeira in Boetia in Assassin’s Creed Odyssey go into the main cave and slide through the hole in the wall behind the steam.
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Keep left and follow the cave until you get to a hole. Jump down and get out of the water. Then follow the cave until you find Deineira.
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She’s at Kreusis in Mount Helikon in Boetaia. It’s like southe western part, near a synchronization poiint.