I'm sure it'll be explained when it gets translated (cough cough wink wink nudge nudge) but for those curious, Akyuu has been shown in Forbidden Scrollery very explicitly praying to Iwanaga-hime, a god of longevity, in hopes of extending her lifespan. Much more recently, Touhou introduced Iwanaga Ariya, who is basically a touhouized version of Iwanaga-hime.

I'm pretty sure this is the first time a character has gone from a generic mythological version referenced in touhou, to an actual 2hu themselves (the closest previous examples being cases where generic species youkai get a representative like Tsukasa). At the very least, it's the first case where a character has a relationship that's such a deep cut into the lore. So naturally, this has resulted in some speculation that Ariya and Akyuu will or should eventually have an interaction.

Another fun fact, but Iwanaga-hime is also tied to Mokou. She refused to let the elixir that mokou eventually drank be dumped into her volcano, explicitly because it was an elixir of eternal life - which fucked everything up because the escort crew was kept oblivious of that fact until then, and it lead to them turning on and killing each other. ...this fact seemed more fun before I typed it out.

C-light said in comment #2590365:

Another fun fact, but Iwanaga-hime is also tied to Mokou. She refused to let the elixir that mokou eventually drank be dumped into her volcano, explicitly because it was an elixir of eternal life - which fucked everything up because the escort crew was kept oblivious of that fact until then, and it lead to them turning on and killing each other. ...this fact seemed more fun before I typed it out.

To nitpick: Mokou and the crew climbed Mt. Fuji and met Sakuyahime, who told them to go to her sister Iwanagahime at Mt. Yatsugatake instead (which they didn't end up doing, because Mokou took the elixir soon after). So Iwanaga herself hadn't appeared in person yet before FW.

(Mokou is also of the opinion that Sakuyahime killed the escort crew herself, unless the soldiers suddenly developed fire powers while she was asleep.)

Are those fumos bottom right?