Yamiyo (a Touhou LW original) refers to a pitch-black night. This is an apt temperament for Rumia, the overly dark youkai. Rumia's temperament has been shrouded in darkness, erasing her weakness to Star Essence.

The ability of Rumia from the world shrouded in overly scarlet mist (a Touhou LW original). It doesn't appear to be significantly different from Rumia's original ability. A suspect has been found for the incident's prolonging. The incident in question is the one where an ominous scarlet mist covered Gensokyo. The shrine maiden and magician decided to investigate. They battled past youkai and fairies in the dense mist to find the Scarlet Devil's mansion, where their desire to resolve the incident clashed with the vampire's ambition. It became the 6th prolonged incident as the mist continued to spread seemingly without end. The person thought to be responsible is none other than Rumia. It would be a mistake to disregard her as a weakling. With her ability to manipulate darkness and her appetite for human flesh, she is a true youkai in every sense of the word. The only thing holding her back is her inability to unlock her full potential... but what if someone else could unlock it for her? The magical darkness Rumia creates doesn't act like regular darkness. It cannot be dispelled with light. In fact, it swallows any light it touches. While Rumia is inside her magical darkness, she cannot see where she's going. This silly interaction makes what should be a terrifying youkai seem kind of adorable... but perhaps this is simply the result of her inability to fully control her powers. In addition to her sight, the darkness also shrouds her other senses, including her sense of self, preventing her from telling truth from fiction and reality from delusions. Apparently, a war broke out in Gensokyo's past, where the youkai split into two factions―one siding with the sages, the other with the vampires. It was called the Vampire Incident. Unfortunately, there don't seem to be any reliable records detailing what actually happened during this incident. All that's been found so far are fabricated reports full of contradictions. However, they are still interesting reads that may offer some insights into the political landscape of that time. According to one such report, the vampires granted extraordinary power to some of their youkai allies, causing them to fight more ferociously. These youkai changed to such an extent that it was like they had been modified on a spiritual level... and Rumia's name was listed among the youkai who received this power. Being weak to sunlight, the great Scarlet Devil enlisted Rumia, the Youkai of Dusk, as her personal shroud of perpetual darkness. In this report, Rumia is described as looking and acting like a tyrannical devil with a personality as dark as the darkness she wields... which doesn't sound anything like this world's Rumia. There are other dubious reports that state Rumia sided with the sages or that during the Heian era (794-1185), a youkai resembling Rumia participated in the Night Parade, but again, there doesn't seem to be any substantial connections between them and the Rumia of this world. It's still a mystery what purpose the ofuda tied to Rumia's hair served. Was it placed there to seal away Rumia's true power, or did it act as a cage for another being? Perhaps it was meant to protect Rumia from that being... Unfortunately, the truth remains hidden. Assuming someone or something did manipulate Rumia into using her powers, they probably only used her darkness sparingly to weaken the sunlight, as that would make producing and spreading the mist a whole lot easier. It would also be much more efficient than enveloping the entire world in magical darkness. However, that would mean there is another mastermind behind the prolonging of this incident... unless Rumia did this of her own volition.