As it was easier for me to just 'make' the ability itself instead of explaining (and it might help you understand how to do it faster

), I made a stand alone map with the ability in it.
All the data that you need has a prefix [CLEAVE], if you search for it, you'll find everything.
Just a small explanation how it actually works:
[1] The ability you use adds a behavior to the caster which will be used to check whether the attack has to cleave or not.
[2] When attacking, the Weapon of the Zergling (our testing unit!) first fires a Switch. This Switch will, with the use of a Validator, check whether the unit has the behavior or not.
[3] If it doesn't have the behavior, the Switch statement will refer to it's Default case: a no-splash damage effect.
If it does have the behavior, it will choose for a splash damage effect.
Everything should be in the map, and I hope I made myself clear. :3 If not, feel free to ask.
