There doesnt seem to be any clear guide as to how to obtain this so far, so I'll include info I used in obtaining this item
In Cape Terrigan, Go to F-7 to a cave, then head to H-5 from there. This doesn't show on the map so just hug right all the way from F-7.Be aware that there are mobs in this area that aggro both sight and sound, right up to level 75, so you may want to either go as whm or /whm for sneak invis, or take along some oils and powders for safetys sake. Find the headstone at this location, and trade a rain lily, for a short cutscene and the reward of a flagellants rope.Please note :In order to obtain this item, you must have previously completed the cape terrigan NM fight portion of ZM5 (which also involves this headstone.