Found in the Grand Archives. Inside the darkened room located after the big wax vat area, look for a lever and pull it. A bookcase will move on the left, revealing a secret room with a corpse holding the spell.
Power Within, when cast, significantly raises the caster's attack rating, as well as providing a large boost to stamina regeneration. This does, however, come at the cost of eating away at the player, resulting in their HP being drained rapidly for the duration.
The overall benefits of Power Within provide the caster with a powerful advantage, but as a result, do not last very long, and use of it will require more strategic thinking prior to being cast, especially if one has a low HP pool and is facing aggressive enemies.
It has a rather slow cast time that can be easily interrupted, and since pyromancy flames do not have a variant of the Unfaltering Prayer skill found on talismans, this will require ample space between the player and the enemy in order to cast the pyromancy successfully.
Since it drains away the HP of the caster, it is very well capable of killing the player if it is cast at low health.