No, there is no spell or anything stupid or nonsensical to become a half demon.
There is no spell to become a Vampire, a werewolf, a mermaid, a spaghetti monster, a arthaomamspdma (made that up just now) or any other non-human fantastical creation you can possibly think of.
It is children like you who are becoming such a Large Nuisance on this site. At one point people came to try and learn something real but now you all want stupid foolish desires not knowing what the hell you are even asking for.
Werewolves, by original lore, can't control their transformation, are very violent and even kill those around them without any control. In movies, those who are werewolves often are heartless and psychotic or feel their "gift" is a curse.
Vampires can't touch sunlight or the day. They need to constantly feed and can only survive by drinking the blood of others amongst other flaws, and are much more monstrous than the embarrassment seen in twilight. Unless you learn have no actual care for anyone or anything and are psychotic or stupid, you desire this path. Watch "Interview with a Vampire", it shows exactly what happens to a Vampire who keeps his/her humanity.
A demon, for the most part, is a very chaotic spirit. If you are looking at the mainstream definition, they are never in peace, always under a feeling and sense of chaos and malice. Basically, there is no good trait to go with this, as everything in their existence is, literally, hell.
Why you would even want such a spell or life is beyond me. Oh, I get it, It's the stupid fantasy that's given from media and you are drawn to some special life. Or perhaps you feel your existence is stupid enough and want to add a little spice.
Stop being so stupid. You want to learn magick, learn magick. People come here looking for something stupid and simplistic. They come to learn "How do I become a (insert the blank)". Only G-d knows why.
Grow up, accept your humanity because it's a great thing. Stop being immature and childish. Come here to learn Magick, come here to learn secrets and grow spiritually. Stop coming here for stupid things when you have no idea what the hell you are even asking for anyway. Come here with some knowledge first also. I'm getting really upset by all the nonsense here, all the idiot people wanting something stupid.
What ever happened to growth? What ever happened to the desire to understand the universe and all it's fundamentals and workings? No, all that's now desired is to be some flashy stupid being because of the romantic view behind it.
Before you even come here looking to become something, look into the damn lore of where these creatures came from. The original Vampires were monstrous beings that did not look remotely human, some only targeting children, drinking their blood and eating their flesh. The first medieval vision of a vampire is a revenant, a decayed corpse that would raise from the grave killing people and feeding on blood. That is your vampire for you, not anywhere close to that twilight garbage you see.
The werewolf is a violent and animalistic being, If I am correct, and had no control of their actions outside of basic impulse. It was often found to be a curse, and is even portrayed as such in most media. Again, that twilight nonsense is far from the remote truth.
As for you and your demon desire. Demons are also portrayed as monstrous and sadistic beings who are in a constant chaotic state. Given there is much more controversy as to what a demon is and how they act but one view is a grotesque entity.
Think about all this. If you have a desire to be different a special, praise your current uniqueness and hone in on your talents that are different from others. Create your unique personality. If you're looking for an escape from your life, play a damn video game, watch a movie, read a book, or change your life in a plausible and realistic way, but stop looking for such stupidity and nonsense.
If I get gagged for this, it was well worth it. Mind you, I was being as kind and courteous as I possibly could when writing this, I feel I did a good job.
Wow, Darkked... You've left me speechless. That's a grandiose post ! No kidding.
Let's just get into it scientifically to make things a bit more clearer.
To be "half" something for us means that 12 of our 24 pairs of chromosomes has a specific origin. In order to contribute to our genetics like that both of our parents must have DNA, therefore they must be physical. Demons for one are not physical and two do not have DNA.
Even if demons were physical beings (they're not this is just for those that might argue and say they are) it's assuming they also have 46 chromosomes that possess the same genes as humans or at least VERY similar, the odds of which given that it would entail at least 3000 years of isolated reproduction are pretty much non-existent, same reason why you can't cross a cat and a dog. If someone's told you they're a half demon and that they suspect you're one too then they're simply role playing, ignore it and move on or they'll draw you down the same path. So to say it shortly, even if they were physical beings they most likely wouldn't be homo-sapiens which means, not being of the same species, there would be the factor of gametic isolation. We wouldn't even be compatible parents.