Blazing kusabimaru?

[hertz]

Today, when fighting Chained Ogre inside Ashina Castle on NG+, I used a single charge of Okinaga's Flame Vent on him, and immediately started attacking.
My blade started burning and leaving a blazing trail just like the weapon of Juzou the Drunkard.

What was it? I couldn't find it in the description of prosthetic

[Sasha Mason]

Living Force is the skill you are looking for. At least I think that's what it is called. It allows you to do follow up attacks with the Flame Vent and Divine Abduction that will cast the effect of the tool onto the weapon.