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Saigyoji's Nacht is a mix (Or a crossover) of Yuyuko Saigyuoji and Walpurgisnacht from "Madoka Magika".
Yuyuko was guilty in snitching spring during the P.C.B events, thus she's ready to do even amoral things to reach
her goals. Walpurgisnacht is... perhaps not the greatest witch, but the most feared one by all Puella Magikas.
You can find more infos about her on the internet.
Naturally it's not possible to win a fight against her, but (SPOILER WARNING) eventually she got defeated.

Reimu, Marisa and Sakuya (I always imagine that they are fighting together side by side) defeated Yuyuko and the
spring came back to Gensokyo.

Saigyoji's Nacht is a depiction of post-bad end of the whole humanity.
Unbelievably strong ghost unites with the sabbath of disaster's, becoming the ... "God"?
Nothing and nobody is going to survive this disaster.
The post-apocalyptical world will dwell with ghosts and distorted minions. Youmu is not going to change that much,
however her human part will begin to decease.
I don't think there is something more to say in general, perhaps some details.
The Lights are ancient and usually require some magic to keep working. Thanks to the Nitori, the lights are autonomous
and use electricity, so there is no need to spend even magic on them.
Yuyuko doesn't get now that it's a pain for Youmu to clean them all, and their light is useless now, even thugh
some time ago the Ghost Princess insisted to keep them.
"If you keep the lights on, it's less likely for somebody to fall. After all, it's not comfortable to walk through the stones on high heels"

• A notice from tengu's notebook.

The Venus flytrap is not a native resident of this garden, it appeared after the "mutation" of Yuyuko.
The origin and the owner of the Katane in the front are unknown. I just liked how katanas are scattered around on some arts
featuring Youmu.
Speaking about her, I had fair troubles drawing her legs. So i just painted them ghost-alike, It's quite acceptable for her now, right?

Walpurgis-Yuyuko got flowers instead of chains, she looks cuter this way and
Saigyou Ayakashi's roots now grow in the infernal mechanism of the witch, (or the ghost?)
The whole thing looks like an amassment of gears, some of them are broken, but the whole mechanism still works.

(https://soundcloud.com/sd_inc/als-saigyojis-nacht-kommt-es-wird-schon-zu-spat-charming-apocalypse)
The track tells you a story of a very few survivors after the apocalypse caused by Saigyuji's Nacht.
About a man, who was hoping to find his family and the others.
Sounds like "Walking Dead?", right?
Oh no, it's more about the change of his mood and condition, and hissing at the end is a message from the minions.
Finally something drags the man into the darkness. No Happy End for today!