Plot plot plot plot plot plot plot plot
Stage 4 boss is
Shiran Tokujakubou ("Priest Shiran"), the leader of the "Youdo Shinshuu" Buddhist sect for bird youkai. (She's not actually Shinran lol, she's just cribbing his style.) Half of her spells are plagiarized from other Chirper users (Aya, Sagume) and her final spell is a spirit bomb based off of that JP Twitter thing where, when the Ghibli Laputa movie's on TV, literally everyone shouts "BALSE" at the same time as the protagonists near the end?? god
Stage 5 is a desert in the Forest of Magic --> big pyramid tower built by General Set. It's called the 禁呪塔 (kinjutou, 'forbidden incantation tower') as a pun on 金字塔 (kinjitou / JP word for pyramids) and the game was originally straight-up going to be called Touhou Kinjutou, but Milka thought that'd be too obvious so she changed the title as a red herring lol. Mystia flew over to respond to the bird gang's request for backup but oh god, she didn't expect there to be a desert here, help
Stage 5 boss is
General Set, inhabiting the body of a Seto-Taishou (
http://yokai.com/setotaishou/).
Set has an ongoing feud with Ra/Horus/etc. as per Egyptian mythology; he came up with a plan to stop time while Ra's boat and all the deities aboard it are in the underworld for the night, thus trapping them there and giving Set free reign over the world. To accomplish this, he stole one of Thoth's magic texts (Thoth being a god of time) and took it to Gensokyo. Since ibises were one of Thoth's main symbols, some of the extinct ibis population now residing in Gensokyo might be able to recite the spells in it. (Also, Set's own body got chopped up at some point during his conflicts with Horus, so he's only a set (ha) of limbs. The rest of the body belongs to the Seto-Taishou lady he's possessing.)
(also milka mentions in the video dialogue that the pyramid is, like, driven into the ground as a symbolic wedge to hold time in place. neat)
Set hits the pyramid's self-destruct button after the fight and we have to ruuuunnnnn out of there!! Escape sequence!!
(more trivia from milka: the fairies in the pyramid aren't trying to escape, bc they heard about how "mummies were preserved in pyramids as preparation for their next life" and telephone-gamed it into "if a fairy dies in a collapsing pyramid they'll be super ultra strong when they revive!!" lmao)
Stage 6 boss is
Horou Torisumi, a rando ibis youkai who Set hired to recite Thoth's spells. She'd normally be a stage 1 or 2 boss, but she's armed with a high-tech Emerald Tablet and the Eye of Horus and currently in superboss mode. She wants to use the Tablet's spells to pass through Gensokyo's barrier and reunite with her fellow ibises in Sado. From her second nonspell onward, she starts forcibly changing the color of the player's options. (also i suspect her 'ibis' and 'book' elements were partly conceived as a homage to Tokiko lol.) Set's time-stopping plan ultimately only extended to Gensokyo thanks to the Hakurei Barrier; Shiran foresaw this when Set approached them with the plan, but went along with it anyway A) to help Horou reunite with her fellow ibises (and then hopefully convince said ibises to convert to Youdo Shinshuu lol) and B) as a big fun event for all the bird gang members to participate in before Set inevitably got defeated by the protagonist.
Also the second S5 boss is Nephthys who's here to drag Set back to Egypt. (i hope she shows up again in the EX stage...)
edit: The game is all done and ready to play btw, we just have to wait for it to be approved by the Freem! upload mods cos it's hosted there this time around instead of axfc