Final answer:
The Spartan king who died fighting the Persians at the Battle of Thermopylae was Leonidas.
Step-by-step explanation:
The Spartan king who died fighting the Persians at the Battle of Thermopylae was Leonidas. He led a small Spartan force in an attempt to delay the invading Persian army and buy time for the rest of the Greek forces to organize a defense. Despite being vastly outnumbered, Spartan king Leonidas and his troops fought bravely until they were eventually surrounded and overwhelmed by the Persians.