Mnemon was a servant of Achillles and was his official 'remembrancer' or 'reminder'. Achilles had been warned by Thetis that if he ever killed a son of Apollo, he in turn would be killed by Apollo. Mnemon's job was to remind Achilles of this - and of nothing else.

But Achilles did kill a son of Apollo, Tenes, who was hurling rocks at the Greek ships. Realising too late what he had done, Achilles killed Mnemon because he had failed to remind him of Thetis' words. (There must be a moral in this story somewhere.) Mnemon's name simply means 'remembering' (mnemon). Compare Mentor, Minerva, and the English word 'mnemonic'.