The Pure Corruption aura is the aura particle surrounding Corruption 2 enemies. You can equip it as a an accessory after defeating a Corruption 2 enemy and getting this as a drop.
The drop chance for this aura is 12.5% on any Corruption 2 enemy. Defeating multiple Corruption 2 enemies in one battle (i.e 4 C2 Bandits) makes your chance 50 (12.5x4). The main chance, 12.5%,is the chance for all enemy drops.