Icon of Chaos is attakcing earth and wants to break the Hakurei Barrier, to unleash, his name sake, chaos.

Play Thorught the entire doom campagin and at the level of E#M8 or MAP30 you get to play a 5 stage short campagin defeating the icon of chaos

Reimu is setn out by Yukari
Marisa is sent out by Alice
Sakuya is sent out by Remilia
Reisen is sent out by Kaguya
Utsuho is sent out by Satori
Youmu is sent out by Yuyuko

Stage 1: The protagonist is sent back from whatever campaing they were fighting to the forest of magic. Confusion strikes until ther "boss" character explains
that the icon of chaos menages to influence some residents of gensokyo. so they thight through the forst of magic and lake of mistrey

Boss 1: Crino is revealed to be possesed by the icon in exchange of in making her stronger

Stage 2: The protagonist is sent to a techbase reminisent of Doom 1, now they are confused why they are flip flopping between dimentions. Their "boss" character explains 
that the demons made deals with some residents to get them out and help them, so we need to stop and return them to gensokyo

Boss 2: it's revelaed Sanae was possesed by the icon in exchange to make her shrine more popular

Stage 3: the player gets sent to the path of reconsidiration, for some reason, there is very slight demonic activity.

Boss 3: Rumia is just there, she is helping the demons out, cuz she is a man-eater

Stage 4: an ash-filled gray part of hell, communication with the the "boss" characters is heavily distorted so they break up, this is a heavily demon infested part of hell, 
and a lot of shadows.

Boss 4: Mimicshade a miciry demon treis to fool the characters by jamming their communications then posing as the people that sent them. then when the jig is up they anger the mc, by turning
into their loved ones, and then the characters themselves to have a mirror match

Stage 5: Hell mimics the persons home, then slowly falls apart revealing a corrputed land, where the final boss residises

Boss 5: Icon of Chaos, massive being made of hellish energy

Stage 6: If all five hover demon parts are collected, the "boss" characters mention there is still some demonic energy to be found, you are in a tech base and assemble the hover demon.
If not all hover parts are connected, the "Boss" characters mention there is still something to be found, and you can replay the levels where you havent sound the parts yet.

Boss 6: Hover Demon, if you can find all 5 pieces you can put him togheter and fight him for fun

Characters:

Reimu and Yukari communicate using advanced telepathic spell that yukari learned

Marisa and Alice communicate using walkie talkies inside the gear she gave marisa

Sakuya and Remilia talk using telepathy from Patchouli

Reisen and Kaguya talk using the magic of the moon

Utsuho and Satori communicate using a more advanced version of her mind reading skills

Youmu and Yuyuko speak using spirit communication

Alice was asked by yukari to gear up Marisa and send her
Remilia was convinced by Yukari to send Sakuya, as if the barrier is destroyed the Gensokyo they know will cease to exist
Kaguya was convinced after yukari told her the atrocities demons would do to her immortal body if they were to break the seal
Satori went easy and boroed her most powerful pet to yukari for use
Yuyuko due to being fimiliar with yukari and understanding the gravity of this situation sent out youmu
Yukari sent out Reimu as she is her most relaiable person

The Hover Demon - Cocky vulgar and pretty dumb, he is a demon that has the huburus of a human

Due to Reimu's personality, he taunts her

He insults Marisa to hell and back

He tried and fails to break sakuyas temper

He taunts with Reisens inability to drive him more mand than he already is

He is intimidated by how large Utsuhos cannon arm is comapred to hi, but quickly discovers she isn't actually demon born form hell and mocks her

He laughs at youmu's phantasmophobia when she is haf ghost and works for one