Recovered from a body lying on a guillotine-table. Located on a somewhat hidden floor of the gaol.
To reach this floor, the player must progress through the dungeon until they reach a jar-elevator that has already begun to move on its own. This elevator will first descend down to a stone bridge (which, when followed, will progress towards the end of the dungeon) before automatically ascending back up again. Jumping back onto the jar-elevator as it ascends and staying on it until it reaches the top will eventually take the player to the section where this item can be found.
Notes
While the skill is technically just a ranged attack that is strengthened when used after blocking an attack, it's best used as a counter against ranged attacks (preferably Physical or Magic ones) since those usually deal less damage and stamina damage, and the shield doesn't have the best blocking statistics.