Hail HU Community.
I've been playing with possible items/abilities and found a pretty interesting one.
As long as my limited knowledge goes, On hit damaging skills at high levels are merely a visual effect for giggles, as an example, Immortal King's level 40 chain lighting on attack. It is, even at 40, a extremely weak ability (due to no Lightning Mastery or Sinergies) and does nothing but hit some enemies with lightning, so you know it did proc. Awesome.
I found one that can be pretty devastating. Ether Nova.
It's the nova baal casts when you fight him on Worldstone Chamber, which sends a bunch of missiles around you, and each time the missile hits something, it has a chance (Maybe everytime it does land, not sure) to cause another Ether Nova to irradiate from the target, which can destroy some weaker enemies and do decent damage to stronger ones very quickly, while being mostly useless against bosses.
The only balance Issue i found was that sometimes, when stars allign or something, you can create a chain that goes on until everything is pretty much dead on the whole zone, or until theres no more enemies to hit in sight. That can easily be fixed by lowering the range, which is around 2 screens from the center right now.
Would make a very nice on hit/struck proc, creating certain chaos on big groups.
Overkill.
P.S.: i can try and take a nice shot of the ether nova in action if needed. But im not at home atm, will be in some hours.
Edit: There goes the ether nova in the Glacial Dome. It wont kill the monters, but put everyone at 50% HP or so, kind of strong for a proc, but could be 1-3% chance and the damage could be lowered.