Thanks so much for playing and we're really glad to hear you enjoyed the game :)
We had actually built a tutorial that walks through the game mechanics, including the special attack, but unfortunately missed the deadline for that build by a few minutes which is a shame.
Once the energy bar is filled it will flash green to signal you can use your special attack. To use it you need to hold down right mouse button for a few seconds, you'll see a large circle growing from within your void, and anything inside the circle will get absorbed when you release. You can risk holding it longer which will absorb pretty much everything on screen if you can risk losing some health during the charge up.