You know it says something about the trend in the anime industry where we see an episode labeled x.5 and immediately think it is a recap. But here the runtime being 12 mins did make you wonder. Tbh this is such a great series I would watch a recap. As this episode starts, and I assume this is Shinobu telling the story. She mentions how this story really happened 600 years ago. She mentions that a story like this with no morals and salvation is better off left as a lie or fiction in other words. That was the piece of information at the jump that gives you the feeling this story is about to be messed up and heartbreaking.
Recalling from Zoku Owarimonogatari, when Koyomi met Shinobu in the mirror world. Ononoki mentioned how Koyomi Araragi couldn't be in Princess Shinobu's presence too much. This story really explains why she was behind a curtain. Essentially, her beauty was beyond praised, but she wanted to be judged for who she truly was. When the witch gave her that wish, the absolute worst happened. Various people killed themselves because the soul outmatched the outstanding beauty she had.
Her father killed himself for not being aware of this. Her mother killed herself knowing she accomplished her role as a mother and there was not much else she can do. Various artists and performing artists killed themselves because their art couldn't do Princess Shinobu justice. I can't imagine how Shinobu felt seeing the pile of corpses.
The most depressing thing is Shinobu was unable to die, and she just watched the pile of corpses getting bigger and bigger. Her only option to limit her suffering was to isolate herself. She did that day after days so that no one would suffer the consequence of dying because of her soul, which can be described as a curse.
This really highlights how much pain Shinobu has been in. Watching all of her people die in front of her. Then having to live in isolation. Then becoming a vampire where her first minion, Seishirou Shishirui tried to commit suicide. It wasn't until she met Koyomi Araragi met Kiss-shot Acerola-orion Heart-under-blade in Kizumonogatari where the start of her happiness could start. Though this didn't start materializing until the end of Bakemonogatari since she wanted to kill herself at the end of Kizumonogatari and this episode even gives more indication why she wanted this.