Baldur's Gate : The Sword Coast Chronicles : NWN2 Persistent World

there are on hit properties, but they trigger every hit... so you are talking about giving bosses spells that would get spammed a lot to the point where you would need elemental immunity or w/e for the spell that is being used to even last a few rounds

but as long as its not spamming grease the server probably wont crash from it

A percentage on hit effect wouldn't necessarily go off every round. Keep the chance low and I'm fairly certain the attack still needs to beat the defender's AC to fire.

I can't quite recall if it was easy to add, but Chaos Shield and slimes have an on-struck effect that would be a nice trigger for adding unusual effects as well.