There are two lightning staffs in this game. One in a secret room in the Vault of Vrenor, and another one in the last dungeon of Chapter 5.

The first Lightning Staff

First, you’ll need the Ultimate Key for this one. You can get it after helping the queen of Femiscyra.

It’s very likely that during your journey you’ll find parts of a diary written by Rowand talking about this staff’s location. You’ll find pieces of the diary in Bath, Porthrunnel, Mintos, the cave to the west of Aubout du Monde, and Femiscyra. 

By gathering the clues you’ll find out the staff is in the same cave where you got the Armlet of Transmutation during Chapter 2. Zoom to Vrenor and head there. 

Go south of Vrenor to enter the cave. Go through it in the same way you did during Chapter 2 (it will be much easier this time), and examine the chest that previously had the Armlet of Transmutation. This time it’ll reveal a switch that you can use to access a secret room. Use the Ultimate Key inside this room to open the cell and get the Lightning Staff inside. 

The Second Lightning Staff

You’ll find this one much later in the game after you reach Nadiria and defeat the four bosses that block your way into Psaro’s Castle.

Once you get to Psaro’s Castle, walk east and south, avoiding the magic tiles that damage you. Get to the next floor, exit this room and take the path north to get inside the castle again. 

From here follow the only path you have, and on the first bifurcation go to the stairs in the small room east of you. Exit this room through the south side, then go to the next entrance and step on the elevator. 

After the elevator takes you back to the previous floor, walk all the way back to where the elevator was. This time you’ll be able to walk over the elevator’s floor to access the next stairs. 

From there head east and south, walk east through the area outside the castle and get inside the room. You’ll find a chest with a Yggdrasil Leaf and another one with your second Lightning Staff.

You got both of them now! You can cast Evac now and head to Heaven’s Haven to recover and keep exploring. 

This staff can be equipped only by Borya or Maya, but if you want to use it as an item to cast Kazap on enemies, you can give it to whoever you want.

As a curious note, the Hell Raiser enemies use Lightning Staffs as an item.