Schutzengel's Oath

Schutzengel's Oath (守護天使の誓い) was a story event in Assault Lily: Last Bullet, starring Hitotsuyanagi Riri and Shirai Yuyu, with Yoshimura Thi Mai in a supporting role. It ran from January 20, 2021 to January 31, 2021, and has been rerun twice in March 2021 and January 2022.

Translation
The complete English translation of Schutzengel's Oath can be found here in Google Doc format.

There is also a full set of subtitled YouTube videos by GEHENA Subtitle Labs.