Did the 300 Spartans Really Exist?
Yes. Leonidas took three hundred Spartiates to Thermopylae in 480 BCE and they died there. But they were not alone: several thousand other Greeks held the pass with them, and around a thousand stayed to the end.
Herodotus is the source and he is close to the events. He gives the three hundred, and he gives the Thespians and Thebans who remained when the rest withdrew (Histories 7).
He also explains why only three hundred. Sparta was in the middle of the Carneia, a religious festival during which the army could not march, so Leonidas went ahead with a picked advance guard while the main force waited (Hdt. 7.206). The three hundred were a holding action, not the whole army.
The epitaph at the pass is recorded by Herodotus at 7.228: go tell the Lacedaemonians that here we lie, obedient to their words.
What the film and the comic add is the rest. The historical account has no monsters in it.
In the Codex. Which is where the Codex differs from Herodotus, and why it is read as fiction by every scholar who has looked at it.
Further in
Acenes of Sparta: The Dragon Slayer
A dragon is the reason the 300 died. Leonidas used the magic of the gods to bind Kalaxes beneath Taygetus, and the price was his life and his men's. Sixty-two years later a seven-year-old of the agoge falls into the dragon's crypt and inherits the unpaid balance.
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