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.
Unique to Highwind XI
In the retail version of the Zeni System, you are required to take photos of monsters using a Soultrapper and a Blank Soulplate. Doing so would grant you a Soul Plate that could then be traded to Sanraku for an amount of Zeni relative to the quality of the photo you took. Because this system has not been fully completed, Highwind XI offers a much more simple way of obtaining Zeni.
How to Obtain Zeni
Zeni is obtained by killing monsters throughout the Treasures of Aht Urghan regions that reward experience points while Sanction is active.