Melee Splash Jewel
This is a new stat that can spawn on jewels(yellow/blue). It makes single-target skills do splash damage in an aoe around the target you hit. This helps make melee builds viable as they will no longer need to hit every single monster one at a time. It does not work with AoE skills such as Whirlwind. Your splash damage is always equal to your Normal Attack tooltip so switch to Normal Attack and open your character sheet to see your splash damage. You can equip a melee splash jewel anywhere on your character or mercenary, not just the weapon slot.
Melee Splash will deal 100% of normal attack damage in a AoE cone when you hit an enemy.
The skill you attack with itself therefore does not add to the splash damage. So for example if I attack with Sacrifice and Might, Might will increase normal attack damage so it will increase splash damage too.
However Sacrifice does not increase normal attack and therefore adds no bonus to melee splash.
Also, On Striking (hit) effects work on the splash AoE but not On Kill or On Attack effects.
If you want to see how much splash damage you do, or what affects the splash damage, simply equip Normal Attack on your left click and look at the damage.