Hoping maybe somebody can recall this old mod...

[Kamen Rider Kuuga]

My memories are extremely vague and sketchy on this mod, as it's been many years. It was a weapon mod, specifically a sword, I think a katana. When installed, the weapon would appear in the Imperial City, I think the Talos Plaza District. I think there may have been a small quest involved, or just text would appear stating that you hear a cry or a voice.

 

When using the weapon, after a certain point, you would get a message stating that you could release a demon from it. I think it added a spell. It was something along the lines of "Yodo Siren", or maybe it was "demon from the sword" or something to that affect, which would summon a female creature for a short while to attack for you, then it would vanish.

 

I think it was potentially based somewhat around the manga/anime Bleach.

 

... I'm at least somewhat sure I'm not crazy and this actually existed at some point...