Defilers can cover enemy's armor with vile toxins, which significantly weakens it, so claws, spikes and other Xerj weaponry will deal much greater damage. Plague will accumulate on enemies after Defiler's attack, and direct attacks that affect armor will consume applied plague to deal triple damage. Plague can stack up to a third of target's armor (all of that armor, except for regenerated later, will take triple damage from subsequent attacks in this case).
For example, a defiler with the plague shot 250 hits a cruiser with 2500 armor. The cruiser has 2500 armor 250 of which are plagued. The next shot made by mudkip has the attack power 10. Mudkip hits the plagued armor and the force of the shot triples. The cruiser now has 2470 armor, and 240 armor remains infected with plague.