Leonidas I appears in Assassin's Creed Odyssey's opening sequence. The game begins with the Battle of Thermopylae in 480 BCE, where Leonidas leads a force of Spartans and Greek allies against the invading Persian army.
After the opening battle, Odyssey shifts to the life of the player's chosen mercenary protagonist, Alexios or Kassandra. Leonidas remains important to the story because the protagonist is descended from him, and his broken spear becomes a significant artifact connected to the Isu and the protagonist's abilities.
Leonidas was a real king of Sparta from the Agiad dynasty. The historical Battle of Thermopylae became famous for the Greek resistance at a narrow pass, though the game's cinematic opening takes creative liberties. Agis II, Agesilaus II, and Cleomenes I were also real Spartan kings, but they are not the king featured in Odyssey's opening sequence.