What the ballista fires is known as canister shot.
Just a barrel wich disassembles mid-air due to the
ridiculous power of those ballistas. Releasing whatever you put into those barrels.
Quite a nifty solution, since those wyverns have been shown to be
nimble and able to avoid single projectiles..
Avoiding a cloud of smaller projectiles is another matter... And while, as shown, each individual projectile doesn't do
much damge, they sure as hell do a number on the wing membranes..
And a wyvern that can't fly is just a very awkward and slow lizard-shaped target for the stuff that can
really take care of it..
After it has stopped going ballistic.. ( minor oversight there...

)
That rider makes a nice smear on the wall though..
