To achieve that, you would still need an ability, but without a dummy unit.
You should follow Pharaoh_'s method for doing it. I'll expand a little bit on what he meant.
"ability based off another ability that already has a buff" just really means an Aura ability.
In whichever ability you chose to be your Buff giver, set the targets to Self and the buff to whichever buff you want it to be. Modify the fields to your liking. Then give that ability to the unit. If you want some kind of SFX to be attached to your unit, edit the Art - Target fields in the ability itself instead of doing it in the buff.
If you don't want the ability's icon to appear in the unit's command interface, you'll need to use spellbooks which Pharaoh_ has outlined for you. Or you could just use Slow Aura (Tornado) for your base ability but keep in mind that the buff will be considered negative and have red text.