Unlocks: Fright Rider's "A Real Drag" Ability
Once you've destroyed the second turret on the Spectral Dreadnaught, you'll be on the far end of the Propeller Array.
Hit the closest switch to lower a nearby gate, then make your way to the newly opened area.
Switch to an Air Skylander to get rid of the whirlwind barrier, allowing access to a warp pad.
The pad will take you to Freewind Flats. Make your way through the zone until you reach this island.
Now just hop up the hill via the spring pad and grab the Soul Gem.