It can start spawning on items at Item Level 18, but will not be able to spawn with maximum effect until level 28.
"On attack" means every time you swing your weapon, "On striking" means when you land a blow, "When struck" means when the enemy hits you.
10% Chance to Cast Level 3 Nova On Striking
12% Chance to Cast Level 4 Nova On Striking
Can appear on: Magic Items, Rare Items
Is available on: Melee Weapons, Throwing Weapons (except Amazon Weapons)