the crystal here is shown in the game

the lore goes that crystals like these suddenly descended from the skies one day and started growing and corrupting everything around them which caused civilization to collapse

people who got infected by the crystal lost their minds +
and started mindlessly attacking others

By the members being put in a room with a crystal this size I'm going to assume they got captured and the antagonist faction (there's a few) is deliberately trying to corrupt them in order to reduce the strength of RoSE which is a private+
military organization owned by the main character's sister ingame. JiU's character is not captured though so she has to hunt the members down and kill her old teammates herself. Don't recognize most of the locations but the background for Siyeon reminds me of one of the early+
story missions. No idea where the main character went though because they're supposed to not only be commanding this squad but also have the innate ability to block the crystal's radiation.
update: I just remembered that JiU's character has a "Corrupter" skin ingame. So it might be that she was the one who captured the members and is trying to corrupt them, but fails and decides to kill them after all.
btw RoSE stands for Ro's Strategy Equipment

Ro is short for Rococo who is the main character's little sister

RoSE is a private military organization whose base of operations is Princess Island which is named after Rococo herself. Though a heavily armed military fortress,+
Princess Island is also famous for being a paradise-like vacation spot, making it unique in the post-apocalyptic setting of Girl Cafe Gun.
update after watching the MV.

it seems that the MV is kind of a mix of deukae's nightmare arc and the Girl Gun Cafe world as a nightmare realm. Fitting, since society has all but fallen in that alternate universe. The following tweets are merely assumptions I have made based on+
what I know about the nightmare arc and the world of Girl Gun Cafe. It seems that the members are once again trapped in a nightmare (the world of GGC) and JiU is the only one who is awake, thus tries to save the other members by going into the nightmare herself to attempt to +
retrieve them. To do so, she has to kill the members in the alternate universe. Each member has replaced a member of Squad 08 of the R.o.S.E. corporation while possibly retaining both sets of memories, which explains why they are able to recognize JiU at all.
With her gun containing only five bullets, JiU successfully kills all of the alternate members thus returning them to reality. However she herself is unable to return due to running out of bullets and thus being incapable of killing herself.
She is therefore left trapped in the nightmare (represented by the crystal) while the other members return to reality. However there's still some unresolved plot points which I'll also attempt to analyze.
Firstly, the crystal. This room appears to be in Dreamcatcher's universe since it is where the other members appear unconscious and where JiU enters the nightmare. The crystal is reminiscent of SOLOMON, the codename used in the GGC universe for the 7 crystals that first fell+
from the sky in the GGC universe that were the main cause for the collapse of civilization due to their corrupting power. It is later revealed in the game that the crystals were intentionally deployed by creatures from another world codenamed "alpha".
With near magical powers like superhuman strength, speed, intellect and yet unrevealed powers, it is not too much of a stretch to imagine that they would attempt to invade other worlds after the GGC world. Therefore the "alpha" transport a crystal to Dreamcatcher's universe+
and the members are unfortunately caught in the event. (At least Yoohyeon didn't cause it for once.)
Second: this butterfly.
According to ingame lore, these butterflies are the manifestation of SOLOMON crystal radiation due to large amounts of it being present in one area. This could suggest that the crystal in Dreamcatcher's universe is now starting to spread and the societal+
collapse that happened in the GGC universe will soon happen in Dreamcatcher's universe as well.
The members dissolving into similar butterflies also might suggest that the members' alternate universe personalities have already been corrupted by the SOLOMON crystals.
Another thing: why do the members react differently to JiU's arrival? Some like Siyeon and Dami stay still and accept their fate while SuA struggles against her chains in anger. I mentioned at the start of the thread that SOLOMON crystals corrupt organic beings into mindless+
near-zombies that mindlessly attack anyone unfortunate enough to be nearby. It would seem that in the GGC universe, Squad 08 has been defeated and the remaining members are scattered and are slowly being corrupted by the crystals. Although the MC/commander of the squad+
is supposed to be immune to the crystals' radiation, I'm assuming they're dead since the members, supposed to be in a squad, are in completely different locations. Therefore we can make the assumption that the girls have suffered differing levels of corruption as shown by the +
level of aggression they display upon seeing another person.
So why is SuA in chains? Continuing on from the last point, the members have been corrupted to varying degrees by the crystals in the GGC universe. SuA is one of the most corrupted since she literally tries to attack JiU on sight. It is likely that, while she was still of sound+
mind, bound herself in chains to prevent herself from attacking anyone. When JiU finds her, however, it is too late and she has already mostly been corrupted and will attack anyone she sees.
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