What is "megistias"

Megistias ( Greek: Μεγιστίας, "the greatest one") or Themisteas was a soothsayer from Acarnania who died in the Battle of Thermopylae. He traced his lineage to Melampus. Despite knowing that death was certain, Megistias stayed and fought. An inscription was written by Simonides of Ceos, a personal friend of Megistias, to honor him.