The blue short sword is the Blue Key

Name: Blue Key
Origin: Umineko no Naku Koro ni
Type: A conceptual weapon, yet shaped as a blue short sword
Powers and Abilities: Forces other beings to doubt about their existence, the one ends being denied by themselves
Destructive Capacity: Specialized for witch hunting, can damage incorporeal beings
Performance: Not applicable
Wielder(s): Dlanor A. Knox
Material or Element: Conceptual weapon
Needed Prerequisite for Use: Nothing

Notable Attacks/Techniques or Alternate Forms:

Blue Truth: Used as a theory, said theory must be denied, otherwise it's considered a truth.


Textual definition in the novel:

The primary weapon used by the official Inquisitors of Heresy belonging to Eiserne Jungfrau.
Like the Red Key, it is a conceptual weapon, but instead of denying the existence of concepts, it is used to create self-doubt and forces others to deny themselves.
For this reason, it is called the Blue Key of Reflection, in contrast with the Red Key of Judgment.
Unlike the Red Key, it is possible to evade and defend against it. However, while the Red Key has severe restrictions regarding how it can be used, the Blue Key can be wielded with great ease and manipulated at will in all situations.

*Dlanor rarely uses the Blue Key