The Power Sword is an item in The Wand of Gamelon.
The Power Sword can be found fairly early on in Zelda's quest. It appears in Sakado, found in the second building of the town. The building is dark and Zelda will need to use a Lantern in order to see. The building is filled with enemy Moblin Spearthrowers and Keese. At the top of the building, Zelda can head through a door where she will find the Power Sword. When Zelda has full power, she is now able to shoot a Sword Beam attack out towards her enemies.
Later in the quest, Zelda can return to Sakado after acquiring the Heat Crystal. If Zelda shows it to Yokan, found at the last house in the second half of Sakado, he will upgrade Zelda's sword. The upgraded Power Sword can shoot sword beams at all times, regardless of Zelda's health.