The Storm Gem is up the big spiral staircase in the Tower of Knowledge. Picking it up is what turns on the Special Attack meter on your HUD — the meter is drawn empty until you have it, and it fires on the R key on keyboard or the Triangle / Y button on a controller.

Where the Storm Gem is

It sits up the big spiral staircase at the top of the Tower of Knowledge. Because the staircase is a rotating puzzle rather than a straight climb, you will usually reach it only after you have worked out the two levers.

The full route up — which lever does what, which rooms come in what order, and the Library underneath — is in the Tower of Knowledge guide.

Queen's Domain Storm Gem: how to fire it

Three details people get wrong:

  • The gem's own description does not name the button. It is R on keyboard, Triangle or Y on a controller.
  • You do not need to be holding a sword. The gem fires on its own, from your inventory.
  • It is a HUD meter, not a weapon slot, so there is nothing to equip and nothing to switch to.

Why the meter starts empty

The meter starts empty and it is charged by defeating enemies — the gem gathers energy the same way magic weapons do. So the Storm Gem only pays off in a fight where you have just killed something. Opening a chest does not fill it, and standing still does not fill it.

That means it is a burst tool, not a sustained one: the practical pattern is to kill a group, then spend the meter.

The gem is also a hard requirement for the endgame of the weapon you already carry. To turn the Holy Sword Endu into the True Holy Sword Akunra you need all five Wind Gems and the Storm Gem recovered, and then you take the sword to the Altar of Purification in the lowest layer of the Deimpo Mausoleum. That whole path is in the Holy Sword upgrade guide.

The locked stretch the tower still has

The gem does not open the tower's last lock. The large golden doors near the gem are opened by a key from the knight Budaen, who is easy to miss:

Look behind the cage in the room where you slot the books, by some candles next to a wall of flesh, next to the bookshelves, a skeleton with a love letter and a chest.

There is a second route to him: jump off the big staircase that leads up to the Storm Gem and dash onto the raised area on the far side. You can also airdash to the half-circle opposite the Storm Gem once you have rotated the staircase.

The room where Budaen is also holds the book slots, so the puzzle there is covered in the book puzzle guide.