(For those interested in the gameplay, I used a clear save I downloaded from the beginning to skip the battles, so you'll need to ask someone else)
Gouen no Soleil is the second entry in the Soleil series and the first one where Lovecraftian elements are included. For those wondering about cameos and characters from other games, the protagonist of this story, Nagare, was classmates with the protagonist of Shirogane, and in some of the endings, you get to see the world and some of the characters of Harukazedori ni, Tomarigi o.
The game starts with the protagonist encountering a girl that looks like the little sister he sees in his dreams, accompanied by a Deep One (yes, those Deep Ones) in a trenchcoat and hat. Upon encountering him, his 'sister' gives him a gleeful smile and he loses consciousness, waking up in a ruined building that seems to have once been his school, trapped in the body of his little sister.
What follows is a bit messy, but he ends up contracted to the weaponized spirit, Rin, who was made from a Black Kirin (a spirit whose role is to speak of the end of the world) and forced to join the Seireichou, an organization dedicated to protecting the foreign world he finds himself in from foreign threats. While he does this in order to find a way to retrieve his body, he ends up seriously helping them out, despite the cold cruelty of the leader (a loli named Mugen) and the general harshness of the world he finds himself in.
This VN is a bit heavy on the gender-bending, so for those who don't like that kind of thing, you might want to avoid it. There are good reasons why the protagonist's sexuality is so... fluid, but they are mostly used to amuse the reader, lol.
Like Shirogane, this game has a relatively high ratio of serious story to slice-of-life in comparison to most VNs, with slice-of-life being almost entirely relegated to basic character development. Unlike Shirogane's protagonist, Nagare isn't a natural philanderer... but he has his own issues, since his soul is gradually adjusting itself to his new body.
Anyway, the story itself is typical Soleil... extremely hard on the characters, full of destruction and apocalyptic drama, and deliberately obscure at many points. Anyone familiar with the typical Japanese take on the Cthulhu Mythos will figure out who is behind the villains of the story pretty quick. However, it is nonetheless a nice, dark little story with innumerable bad endings and two separate paths to the various heroine endings (Ouka's arc contains all the heroines except Rin and Ruru).
Don't expect deep characterization... this company generally doesn't waste that kind of effort outside the main heroines of its games (Rin and Ouka in this case) and the protagonists. As such, the endings other than that of the two main heroines are perhaps not as powerful... but I liked some of them, at least... especially the threesome ending with the two Valkyries.