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Help me in understend this card.
My oponent cheat me ?
When I trigger this action, all later corrupted unit is damaged...
or I good understend ?
I trigger this action only when I choose... and damage only choose corrupted unit ?
Sorry for my not fluently language... 
If you control this card, you trigger its Action only when you choose, but also only in response to a unit becoming corrupted. It does not create a constant effect that damages all future corrupted units.
So you played it correctly and your opponent is wrong.
Keep in mind, this card does not trigger off of Warpstone Excavation