Black Demon Pedestal
The gold-trimmed wooden chest where each Black Demon rests, blade planted in the lid.
The Black Demon Pedestal is a dark wooden chest with gold fittings, in which the weapon is planted blade-down, slightly tilted — like Asuramaru beneath Shibuya High School in the anime.
Use
Right-click an armed pedestal: the Black Demon goes into your inventory (or drops at your feet), with a distant Wither breath and a Netherite-armor sound. The chest stays in place, empty and dark (it gave light level 9 while it held its weapon).
The seven pedestals
The 
armory's vault holds seven, one per weapon: 
Asuramaru at the end of the aisle, and on either side 
Kiseki-ō, 
Byakkomaru, 
Raimeiki (on the right as you walk in), 
Gekkōin, 
Mahiru-no-Yo and 
Shikama Dōji (on the left). Each one stands on a blackstone slab, with two lit red candles and sacred cords stretched above.
See also: The Black Demon series.
See also
The Sealed Armory
Step-by-step guide to the Imperial Army's armory: the ruin, the shaft, the talisman corridor, the weapon hall and the Black Demon vault.
The Black Demon series
The seven weapons sealed at the bottom of the armory: Asuramaru, Kiseki-ō, Gekkōin, Byakkomaru, Mahiru-no-Yo, Raimeiki and Shikama Dōji.
Asuramaru
Black Demon of the series. The black katana with a green edge and golden mountings: Yū's weapon and the vault's centrepiece.
Ofuda
A paper talisman: up to four per block, torn off with right-click to open the sealed gate.
Kiseki-ō
Black Demon of the series. A jagged black blade, fast, trailing a cord: the quickest of the Black Demons.
Sealed Gate
An indestructible gate covered in ofuda: when the last one is torn off, it sinks into the floor.