First, make your dummy spell based off of channel, and make sure to set "disables other abilities" equal to true, and set "follow-through time" equal to the duration of the spell, and set the type of spell to "point target".
This will make the caster not able to move once the spell is cast.
For the actual "AoE" effect, I believe it would be easiest to summon a ward there, then use the "move unit instantly to point" to move the unit in a circle.
For circle movement, this requires a bit of math. If you are using GUI, I suggest you use polar coordinates to move the unit however many degrees per second you want. Since "Wait Game-time" is not accurate for time increments needed to make a smooth motion, you will need another trigger to make the unit move in a circular path.
At the end of your trigger that summons the ward, store the location of the circle center and the ward itself as a unit to global variables. Create another trigger that will be disabled at map initialization, that will use the event "periodic timer: 0.1 seconds". This periodic event will be turned on by the same trigger that summons the ward. The periodic event will use global variables to move the ward in a circular path. At the end of the spell, the periodic event trigger will have a condition causing it to disable itself.
If any of that doesn't make sense, or you need some code, please let me know.