Alexander doesn't know what the Forbidden Power is, nor does Tony. The only ghost who is known to have the Forbidden Power in the entire Dark Hell is big brother Louis.
In addition to this, the former head of the Ghost Apartment once tried to peep into the Forbidden Power, but it brought about divine punishment, nearly destroying Ghost City.
Fortunately, Louis had a bit of the Forbidden Power, which allowed him to break the divine punishment that the entire Dark Hell couldn't deal with.
Forbidden Power, as far as Alexander sees it, should be something stronger than ghost energy.
Similar to humans to ghosts.
Without ghost energy, no matter how capable you are, even using a nuclear bomb can't hurt a ghost, just as if this is some kind of unchangeable setting, rule.
Obviously, the Forbidden Power is a similar thing for ghosts.
