The Faint Corruption aura is the aura particle surrounding every Corruption 1 enemies. You can equip it as a an accessory after defeating a Corruption 1 enemy and getting this as a drop.
The drop chance for this accessory is 12.5%. As with any enemy drops, you can defeat four C1 enemies in a single battle and make the chance 12.5x4, 50. The chance changes as many C1 enemies you defeat.