As I suspected, Yukari is possessed by this vengeful spirit.
Now then I’d like to propose a question. Koishi. Where do you think she is right now? This may seem like an irrelevant out of nowhere question. But really think about it. Koishi does not think, she merely acts. Don’t you think this would make her rather easy to possess? Not to mention the fact that Koishi herself is essentially invisible, nobody outside of a select few actually remember her even if they do see her.
So what does all of this have to do with the manga? With the vengeful spirits? Simple. My theory (while about as likely as Mima coming back) is this. Koishi might just be the one spreading around these vengeful spirits, whilst being possessed herself. Reaaally think about it. A girl who nobody can remember nor can they really see. Wouldn’t somebody like that be the perfect host for an incident like this?
If it's Mima, why does she need so much yokai energy to manifest herself?
Does she know that Reimu would exterminate her because it is against the law for a human to become youkai. And so she wants to come back strong enough.
Wait, if the Vengeful spirit really is going after youkai and draining their energy, how much energy would she get from Yukari of all people? She's Gensokyo's most powerful youkai and has lived for over a millennium; correct me if I'm wrong but, if most of the other victims' energy levels were like enough electricity to power several houses, Yukari would probably have enough energy to power an entire city.😶
Also... @charkan, Mima wasn't human to begin with. Yes, Reimu won't hesitate to kill a human trying to go youkai,
but in that case (I'm assuming you're referring to the Fortune Teller from FS) I believe that was also partly because Kosuzu was hurt because of his actions,
but if it WAS Mima, not only is she NOT human to begin with, but it's most likely that she's trying to... re-manifest into a more human form and needs a butt-ton of magic energy to do so.
@Lichzim (take what I say with a pinch/tbs of salt), Koishi doesn't think because she closed off her mind. You'd be correct in assuming she'd be easy to possess, but It's pointless trying to possess someone who can't think to begin with; they can't be controlled. The only reason why she was subject to Perfect Possession was because that phenomenon can be forcibly achieved by catching them of guard, which meant that Koishi could both possess and get possessed easily. However, in this case a victim would drink the "cursed" drink the current host gave them, lose consciousness and the spirit would later take control of the new host. But, unless said spirit was a tsukumogami, it's pointless possessing something/someone without a mind. Possessing Koishi would be like dividing by 0; it's an unsolvable paradox. Not to mention that Koishi acts almost entirely off instinct. Also, not everyone can forget her that easily; if they know about her through Satori (or rather, that she's Satori's lil' sister like Reimu or Marisa) people will remember her.
@McAlastair
I know that I like to freak about "It's MIMA TIME" and shit but the last time we hear about her she decided to become a god by other means besides ultimate magic and her hatred towards human race (whatever was her reasons, ZUN never told about) were diminishing by then