decrease armor on every attack buff?

[codart]

Hi every body, I want a spell which create a buff on enemy for x duration. When ever buffed enemy is attacked, it's armor will decrease by y amount. This effect it ultil buff's duration is expire. How can i do that?
Srr 4 my bad english

 

[ALiEN95]

abilities>edit>find and type "corruption"

press Ctrl + G until you find the ability
(i think it has 3 Data strings)