Flashing Patterns Bullet Hell

Lapis was having a normal day when she noticed a mysterious figure watching her wherever she went. She confronts the figure, she learns that the figure's name is Sellvania and she was from a different world. Sellvania outright tells her that she is an Apostle of the Calamity God, Rena and that she has a target over her head.

So because Sellvania has taken a liking to her, she says she'll protect her. Unfortunately, Sellvania is a mentally insane Reaper and thinks nothing about killing people, and more people from her world are mysteriously coming to Earth against their will, and the government made two dangerous weapons which are now on the loose. And her sister somehow finds herself trapped in another world as the resident Demon Lord.

In other words, Earth has a bunch of superpowered war-crazed beings and it is up to Lapis to try and stop them from accidentally blowing up the Earth. Oh, and Lapis's sister is trying to adapt to her situation being the new Demon Lord.

This is pretty much an R-rated version of Touhou with the beings from another world using energy bullets to attack...or in other words, they attack using bullet patterns.