hmm search in spells for Terrain Spells. I don't remember the author but there was a slime leaving green poison behind, very cool effect. But coming to your spell. The easiest way is to make a trigger as periodic event every 2 secs or so, and pick every unit in range of 200 in behind of the unit, create dummy unit and slow them.
I was trying to make a trail of like traps that slowed units and damaged them, for the damage I just used an immolation spell not sure how I can slow though still.