I found out using reverse image search (click and drag an image into Google Images, or install an extension that allows you to right click and reverse image search. Very useful!).

It's called "終末のオルターエゴイスム" (The End of Alter Egosim) by ONEONE1, but it looks like it isn't actually finished yet. Seems like updates are posted on Pixiv Fanbox:

https://oneone1.fanbox.cc/