Once they added PoK Library spell merchants that have EVERY spell (up to a certain level)
from everywhere, either from other merchants or quested, you can now buy this spell.
"Spell: Concussion" is the store bought version of the quested "Scroll of Brain Bite,"
either way your spellbook will then contain "Concussion." This is a Kunark era
spell quest, so quest for it where they won't kill you (The Overthere or Firiona Vie).
I did this ages ago and I remembered it when I recently ran into the quest skelly in CoM.