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In around 431 BCE, he enlisted the help of the Spartan misthios, Kassandra, to retrieve Penelope's prized shroud, which had been stolen by bandits. He directed her to Odysseus's Palace on Ithaka to start her search. After she returned with the shroud, he gave it to her as her reward. 
Elpenor's next task for Kassandra was to hunt down and kill the Wolf of Sparta, unbeknownst to her, was in fact her father, Nikolaos. The misthios later rendezvoused with Elpenor in Phokis, presenting Nikolaos' helmet as proof of her kill, whereupon he revealed he had known all along of Kassandra's relation to the Wolf. Upon confirming that Kassandra's mother, Myrrine was alive, the cultist attempted to enlist the misthios to assassinate her as well but this only angered Kassandra, forcing him to summon his guards to delay the misthios while he made a getaway.
Kassandra later learned of Elpenor's allegiance to the Cult of Kosmos from the Pythia of Delphi and resolved to track him down. Subsequently, Elpenor devised a trap by placing a decoy at Pharsalos Fort in the Valley of the Nymph but Kassandra killed the decoy anyway and escaped. Furthermore, Elpenor left a letter on his decoy, detailing his orders to kill the "so-called messenger of the gods" and meet him at the Snake Temple in Phokis. When confronted once more by the misthios, Elpenor admitted that Myrrine was the Cult's only concern and fought against Kassandra, but ultimately lost and was fatally wounded. With his dying breath, he declared that Kassandra had made a mistake killing him, to which she retorted trusting him in the first place was hers. He refuted that the cult desired her death and his actions spared her that. As she tries interrogating him about the cult and their location, Elpenor merely chuckles before expiring. His death was later discovered by Deimos, who then decapitated Elpenor and brought his head to the Cult's meeting as an annunciation of his death, which indirectly resulted in the grandchildren of Leonidas learning of each other's existence.