Spell that does three things

[DeathScythe]

How cud you add slow to an ability like impale rather than stun

 

[Eusira]

Dummy casters? Basically spawn some unit with a perfect slow ability, where it has no cooldown, cast time, mana cost, etc. Creat it when the unit starts casting, wait a sec to get all the units on the ground, then pick all units within 1000 of the caster, check for buffs, then slow them. After that, remove your dummy

 

[DeathScythe]

sample

cud u give me a visual example of what the trigger wud be like

 

[Dr.Nitro]

Give a dummy unit thunderclap with no special effect, then do the trigger as follows:
Event:Unit casts an ability
Condition:Ability being cast=Impail(or w/e ability you're using)
Actions:Create 1 Dummy Unit at position of triggering unit facing default building degrees.
Order last created unit to Human-Thunder Clap.