To unlock the No More Rulers trophy in Assassin's Creed Odyssey, you must first purchase the first episode of The fate of Atlantis (or own the Season Pass) and download Episode 1.
There are three Overseers in Elysium that will be highlighted on your map. Once you kill all three of the Overseers, the trophy will unlock.
Iphigenia the Sacrificed:
Overseer of Pheraia's Retreat (Iphigenia's Compound) [East]
Aegea the Amazonian Queen:
Overseer of Deukalion's Heritage (Fortress of Three Sisters) [South]
Atalanta the Huntress:
Overseer of Minos's Faith (Mausoleum of the Kingless Queen) [West]
Not required for this trophy:
Persephone, Queen of the Underworld:
Ruler of Asphodel Fields (Persephone's Akropolis) [North]