Particle of the Outsider is special item found only in New Game Plus. These particles are sold for only 1000 gold each and are used to upgrade weapons and armors further to 99+. They can be found in treasure chests, dropped by enemies, fighting bosses, or replaying a very hard dungeon (Bandits' Warehouse does not count). They are shown to be extremely rare when it comes to farming from enemies. Haunted Mansion is the best dungeon to farm them.