No Spartan commander captured Demosthenes at Pylos in 425 BCE; Demosthenes was an Athenian commander who helped capture Spartan soldiers at Sphacteria.
This question tests a common reversal of the episode. During the Battle of Pylos and the following fighting on Sphacteria, Athens trapped a Spartan force on the island. The stranded soldiers eventually surrendered to the Athenians.
Demosthenes commanded the Athenian operation on the island, while the Spartan commander Epitadas was killed during the fighting. The Spartan survivors were taken prisoner rather than capturing Demosthenes.
Because the requested answer is not a historical person or event, this item is unsuitable as a valid trivia question with one canonical answer. The episode is best remembered for the unprecedented surrender of Spartan hoplites to Athens.