User blog comment:Crimson and Darkness/Shin DxD 3 discussion/@comment-39047410-20190819232412/@comment-35679297-20190823010647

About the movement Tail Blade, in reality Issei stores a third sword Inside his tail, a prototype of Ascalon II less performant than Ascalon II, I have found this phrase on the japanese wikipedia page of DxD which confirms this :

また、アスカロンⅡほどの性能はないが、それでも引けは取らないという試作品も提供されており、本戦1回戦後に鎧の尾へと同化させ、相手の虚を突く3本目の腕として使えるようになる