The Resplendent Night of Azure is a phenomenon native to specifically Kagerou, although it can occur within other reasons. During the night, Kagerou's skies become stormy and cloudy, and charged with static electricity. The electricity is not unlike that of the weather in Bastille Icaria's capital. The night is said to act much akin to Walpurgisnacht, boosting the magic of sorcery and magic users, although Gods can still call upon their power, albeit not as potent. Amongst that, the Resplendent Night of Azure always occurs in a full moon. And any worlds linked to it will have an eclipse, predicted or no. It's known that any worlds linked, however, will be under Walpurgisnacht instead.
Solar executing his Walpurgisnacht spell card during the Resplendent Night of Azure will be not as much as significant aside to only the moon eclipsing, and any communication with Gods, safe for Tsukuyomi, will be rendered silent. It will fully prohibit the usage of any sort of divine instrument or power, unless they're aligned with Tsukuyomi (or an actual God present, that is worshipped in Kagerou or Gensoukyou). Providing a stark different towards Walpurgisnacht in a base. It will also rain were this to ever occur, although the static electricity and the azure aurora will still be present amidst the sky.
As a Concept
The Resplendent Night of Azure would be a plot behind the case were Solar to create an incident in Gensoukyou, this concept had then evolved to a Touhou fangame, which was never really made (as it'd violate the naming rule).. While the story would undoubtedly end in his defeat or stalemate by the person dealing with an incident, the only instance where the draw would be put were if it was either the first time completion, or if the player character was Reimu Hakurei or Reisen Udonge Inaba. The particular reason with Solar dampening Reimu's powers. The latter being familiar with Lunarian, and Lunar Rabbit combat, something which Solar is used to. This is mainly to encourage playing through the individual characters. Any sort of victory on Easy, or a game spent with a continue, would result in Solar's taunting to the player character, asking "Come on, are you giving it your all!?"
The routes would depend on the character, although the character would check amongst the general direction of the Tengu Mountain, then to Scarlet Devil Mansion for Patchouli and Koakuma, along with the Lunarian exiles. Eventually, one or two of the three groups would tell the player character of Solar and his younger siblings (barring the middle child), and would say that they're ultimately responsible for the case. It is during only the true ending, that upon Solar's defeat, Yorihime would arrive and apologize for his doing. She would mention Kagerou before leaving, saying that she'll thoroughly discipline him for his actions, or at least give him a good scolding if he manages to still put up a fight. To the other player characters, they would be quite certain.
During the stalemate, Yorihime arrives to stop the fight in the nick of time, saying she wish she wouldn't have planted her boots on Gensoukyou. She takes Solar away, stating the same thing and doing the same action, although makes it generally clear that his doing isn't the Lunarian Republic's own doing. She would also state that the only time Solar and his siblings would visit Gensoukyou is to visit Patchouli, which is in turn revealed to be their mother. Otherwise, for Solar, it would be to also have drinks and conversations with Aya Shameimaru. He wouldn't generally be present in Gensoukyou's other areas, making him a rare sight, and definitely a person who's made himself scarce.