Yeah I'd have to agree with raft, this sounds like an orb effect to me. I'd go about this by modifying Slow Orb, since it has the % chance thing you were talking about, and changing the ability it casts to something like poison orb or something of that nature. Alternatively, if you don't want to have the item have an orb effect (since they don't stack) Altazen's trigger looks fairly clean. Keep in mind though that the Unit - A unit is attack event is very sketchy, since the event fires BEFORE damage is taken into account. The only real way I know of to get by this problem is using JASS, and being the complete and utter failure I am, my knowledge is extremely limited on the subject. I'm sure however, that some very kind and wise member of the Hive that knows the language would be more than happy to help you (for a couple of rep points of course!)
Anyways, hope this clears things up a bit, and GL with your map!
-NS