This is genuinely great.
It's somewhere in style between Raptor: Call Of The Shadows and Tohou, and it innovates on the shmup style in a number of interesting ways. Enemies take cover. Environments are cluttered and destructible. There are a lot of elements packed into the (relatively) short game that made me think and strategize more than I'm used to when playing a shmup.
It *is* a bullet hell, and you're probably going to want to use the mouse for it, but also your sprite is large and can get kind of cluttered with allies, so screen clearing is a bit more effective than dodging.
Story-wise, there's clearly a plot, and there's some animated cutscenes, but the narrative isn't so much what you're here for. Monsters oppressed. Humans oppressive. I don't dislike the worldbuilding Demonizer is doing, but it's either too present or not present enough. The campaign mode's neat, but I felt like there was some backstory I was supposed to have read for it to have hit home.
Other than that, my biggest complaint about Demonizer is kind of just a complaint aimed at shmups in general. It has win-more mechanics. As you eliminate enemies, you gain blue and red hearts. Blue hearts increase the power and width of your beam. Red hearts add sidekicks. Both have the potential to go away when you take a point of damage---they scatter like sonic's rings, and you have to retrieve them without pathing yourself onto another bullet in the process.
In early stages, you can recover from this. There's enough low-level mobs that you can kind of figure out how to ramp up before the damage sponges and pellet-spammers start showing up. In later stages though, oof. Especially with things sometimes hitting and killing you without it being clear a) what hit you, or b) why it killed you. For example, burning patches on the ground seem to kill you, but only sometimes. And only some types of burning patches. Also water sometimes damages you for flying over it, with no visual indication as to why.
I'm not especially good at these types of games, so if you're a veteran your mileage will absolutely vary, but to me Demonizer felt like something you play through in one successful run or not at all. And so to that end, I kind of wish it had some RPG mechanics stirred in. If you could spend your exp at the end of each stage to upgrade, and you could keep your upgrades, dying once during a stage wouldn't mean having to instantly reset your run.
Still, even with its difficulty, Demonizer is a lot of fun to play. The controls handle well. The visuals are interesting. The enemy patterns and tactics as neat as heck.
It's definitely worth checking out, and I'd love to see a follow-up.