As seen here.
This is a ROM hack of Pokemon Emerald that replaces the monsters with Touhou characters. Instead of being a graphics swap like Moemon is, everything has new typings, stats, and abilities, along with some new moves and a modified type effectiveness chart. This is perfectly playable with no Touhou knowledge, and the new type chart breathes new life into an old region. Additionally, the 4th gen physical/special class split exists along with icon notation, so there's no guesswork involved for a move's damage type.
The game is fairly difficult, but not in the level grinding sense. You don't need to grind to level 40 for the third gym or anything like that. A lot of earlygame foes are able to be overcome through resists, so I encourage captures and experimentation before you try to start level grinding. The main gameplay progression is the same as Emerald, with the addition of a couple of new events. The Battle Frontier, Battle Tents, and contest halls are fully modified, so they're available for your amusement.
Evolutions are different from normal Pokemon in that nearly everything can behave like Eevee - most puppets have a level-based evolution and one or two stone-based evolutions. All evolutions are final, so think carefully about using stones. Forms are better explained in the text files, but the prefixes you may see are:
C - Chibi (unevolved)
A - Attack form
D - Defense form
T - Technician form
S - Speed form
H - Helper/Support form
Ad - Advent form (a special version of Attack/Defense forms for some characters)
If there's no prefix, then it's in its regular evolution form.
To play this, you will need to patch over a clean (U) ROM of Emerald.
There are two patching methods available, Lunar IPS and beat. Links to patchers are supplied in the folders. You'll know the patching worked if the title screen comes up purple instead of green.
When using VisualBoyAdvance, make sure that your save type is set to Flash 128k and that Real Time Clock is enabled, or else the game may misbehave (such as not being able to properly save data.) It will run happily on mobile GBA emulators as well.
There's a significant amount of text files to help you enjoy the game better, so if you're lost, try those out.
Lastly, here's the screenshots I supplied on the main page for it, but they're fairly old. I've since decapitalized the majority of the script and modified a lot of menu palettes.