"An item that absorbs electrical discharges."
The Lightning Rod is a quest item that Geralt must have forged by either Malcolm Stein in the non-human district, or the Order armourer to complete The Sentry quest. It can't be forged until needed during the quest The Sentry. It is one of the few quest items that has to be paid for. The other notable quest item which must be purchased is a sephirah (Tipperath).
Price
Craftsman Price Malcolm Stein 50 
Order armorer 70