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.
The amount of zeni that a pop item costs varies with level of demand (a player purchasing a pop item will cause its price to rise by 100 zeni, and it will gradually drop back down over time) and with the tier of ZNM (higher tiers cost more zeni). Following a server maintenance, the price for all pop items is set to max; however, it can go higher than the max listed.
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 Key ItemMaroon 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 Key ItemMaroon Seal will be lost. In order to purchase another, you must turn in another trophy item that bestows a Key ItemMaroon Seal.