This liquid is harvested from a malboro's mouth. Anything exposed to its rots in a matter of moments.
Foul Liquid is a loot required to create the Khukuri, Giot Gun, Supernal Ray, Battle Folio, Mage Manual, Urutan Annals, and Giant's Helmet. It can be obtained by completing the missions "Pirate Attack", "The Next Step", and "A Hard Place", and is a loot dropped by medium-leveled monsters. It sells for 200 gil.