Zeni Notorious Monster (ZNM) events are spawned by trading a pop item to ??? around Aht Urhgan. The pop items are obtained from Sanraku at (E-8) in Aht Urhgan Whitegate in exchange for zeni. When spawned, the pop item is lost and the monster appears. A full alliance of 18 can engage these ZNMs using one pop item. These are normal battles, not battlefields.
After defeating a ZNM, a "trophy" item bearing the defeated ZNM's name is dropped. It can be taken back to Sanraku, who will give you a colored seal key item in return. The seals allow the bearer one chance at purchasing a higher tier item (zeni is still required for purchase). These are temporary key items and once you purchase a pop item from Sanraku for a higher tier ZNM, the lower tier seal is lost. For example: If you kill Cheese Hoarder Gigiroon in Tier 1, you will get a Maroon Seal upon turning in Gigiroon's Cape; now you want to kill Iriz Ima in Tier 2. As soon as you purchase the pop item for Iriz Ima your Maroon Seal will be lost. In order to purchase another, you must turn in another trophy item that bestows a Maroon Seal.
How to Obtain Zeni
Zeni is obtained by trading in Soul Plates to Sanraku, who is located at (E-8) in Aht Urhgan Whitegate. The amount of zeni rewarded is loosely randomized between 100 and 200 per Soul Plate. You may trade in a maximum of 10 per day.
How to Obtain Soul Plates
This feature is unique to Highwind XI. Soul Plates are obtained by killing select NMs and HNMs that are of value to Sanraku while carrying the Soultrapper and a Blank Soul Plate in your inventory. These items do not need to be equipped to receive a Soul Plate.