When worn, if you lack poison resistance, the cloak delivers a poisoned attack with a 1 in 3 chance of being fatal, otherwise doing normal strength/hp damage and identifying the cloak. If you have poison resistance, the cloak has no effect, but gives the message "This cloak feels a little itchy." Although this unambiguously identifies the cloak, it does not actually self-identify in this case. The cloak does not cause any sort of continuing damage, but only hurts you when you wear it with the W command.