The Queen's Domain Pandora Blade is a hidden sword in the crypt room that holds the Endu Shrine Gate Key, in the first dungeon below the starting camp, and it is still there after you beat the first boss. Its starting Power is only 5, among the lowest in the game, but its secondary attack is a forward dash you can chain repeatedly while falling, and its damage bonus climbs to +300%, which lifts its effective attack value to 20.
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Where the Queen's Domain Pandora Blade is
Return to the crypt room that holds the Endu Shrine Gate Key and look for the broken stone pedestal where the brazier is missing. The wall to its right is illusory, and it opens with the interact button at close range. The blade itself is in the chamber of skeletons behind that wall.
Two details make this easier to repeat. It is a separate pickup rather than an evolution of the Rusted Short Sword, so nothing you level can spoil it. And the crypt is on the same loop as the wooden club and the crypt key, so you can clear the whole opening area in one trip.
The early weapons guide has the crypt route that leads to it.
What the chained dash actually does
The blade's secondary attack is a forward dash that you can chain repeatedly while falling, carrying you farther horizontally than an ordinary jump. On PC the secondary attack is right-click by default.
Combined with the Holy Sword Endu's air dash, that reach grows further. A jump that normally clears a ledge can be extended into a gap you would normally need a different route for, and a fall can be turned into a long horizontal move instead of a drop.
The sword jump guide has the timing that makes chained dashes work.
Why keep a weapon with 5 Power
The blade has an unusually high growth limit. Its damage bonus can climb to +300%, which takes its effective attack value to 20, and it does not evolve at level 10 the way most weapons do. Its level ceiling sits far above 10, so a blade you keep through the whole game keeps scaling while other weapons top out.
The practical reason to keep it is mobility rather than damage. Slot it in as your movement weapon, leave the spear or the club for the fights that need them, and use the dash to open routes that are otherwise closed.
The weapon evolutions guide lists the weapons that never evolve at all.
When to pick it up
Take the blade the first time you clear the crypt, before you have the Holy Sword Endu, because the dash is already useful without any air dashes and the room costs you nothing extra on the way to the wooden club.