I just want to point out something important that I didn’t see mentioned. I can't diagnose your kid but give you some advice from another point of view: The little sister who got beaten up everyday by the aggressive brother.
Your role will not only to help your son but also to protect your other children from him. So remember this when you wonder if you demand too much from that aggressive son.
You can't fix this alone. Get help from an experienced therapist.
Your husband also needs to be present, and you might need to take a few steps back sometimes.
What changes at 21? Adult consequences... That makes self-interest kick in quick.
I also doubt the violence just started. I suspect you simply are the victim now but your son had other targets before that are now not tolerating his behaviors.
I don’t know how old are your other children but your son is almost certainly abusing them as well, especially if they are younger, have a gentle personality or are on the aspi/autistic spectrum. This is really not to be taken lightly as kids who grow up with that kind of sibling often develop themselves issues later in life and post-traumatic stress disorders even if they seemed to cope well.
The manipulation also has a very negative effect on the center of language in the brain and perception of reality, and it's even worse on kids who are actually learning all these social skills. Call it brainwash because that is truly what it is and a young kid can easily mentally overpower a younger one.
A violent sibling can also create that issue in the other vulnerable siblings by abusing them. They too might develop acceptance and anger issues, and use the same coping skills the older sibling used on them.
Violence of all kind should never be tolerated, no matter how angry your son becomes. Violence takes away self-respect and choice. By complying to that aggressive son, you are telling the other kids they need to accept the violence from people they love. As it's not something you can quantify, it opens the door to any abuse by anyone who will act like the abusive brother. Some people here gave you very good advice to make sure you are respected as a parent. Being assertive is better than a hug sometimes and have lasting effects.
The other side of this is not only the manipulation, the psychological and physical abuse (and maybe sexual, which is not rare at all) from your son but also the fact you may end up making that son "your project" and being less available/tolerant for your other children. And draining yourself in this while it might be possible you cannot do much no matter how much you love your son.
Observe your pets (if any) for fear reactions, avoidance, wounds, weird accidents, deaths, "bad luck", etc. Ask questions to your other kids without showing your goal (how they feel, how they play when with the other son, if secrets, games with pets, etc.) Kids have information they think you all-seeing parent know but you don't. My brother killed hundreds of pets in secret and he was carefully planning his "hitler episodes", which I unfortunately witnessed sometimes. My parents only knew about it when I told em when I was 30yo.
If you feel your son has little empathy, you might not be wrong. But empathy can be developed to some degree and it's a choice. It needs work. It cannot be forced. It needs to be rewarded. It needs to become his new currency (what he wants).
One very important thing is to read about shame and control. The episode with the other kids' party has certainly more to it and you don’t know it. If your son lashed at you the way you described, maybe he did something he felt ashamed of in front of his friends but knowing they would not tolerate his anger (but you will), you got it instead. Your son knows he can lose his friends and get a bad reputation at school because he probably experienced that consequence already and didn't find it a "winning move", but he knows he can’t lose you. If your son is really angry and even has physical signs of being in the "red zone", this isn't simple manipulation for attention.
Maybe try to see if anything he might feel ashamed of triggered him when he goes hysterical. It’s most likely perceived shame from his own perception of reality, eg. it can be something as small as not making a good joke or not getting the expected reaction. So don’t take too much pity on that shame he may have felt, it is necessary for kids to process these feelings to develop empathy and it cannot be forced. Your pity will only enable him to escape this process and even create anxiety because it shouldn't be a scary process. Give him acceptance, some freedom to think about it and cool down, and don't make a big deal out of his mistakes but don't reward them either. Apply discipline, offer consistency and acceptance, not compliance. Your therapist will teach you what mindset to have. The anger, aggression and tantrums from 0 to 180 are often coping skills some kids who lack "emotional training" use to get some biochemical "high" that takes away the feeling they want to avoid dealing with. You cannot reward or accept these coping techniques, it's a terrible habit that is very similar to what a junkie would do.
Go see a therapist alone first (or with your husband) then with your son. Find a man. Your son has no choice, he will need therapy. Think that he will only refine his techniques as he grows up but will keep the same attitude and lack of respect towards others. You will not fix that problem alone. Don't shop for therapists with him, go alone first. If you shop for therapists with him, he will totally lose respect and trust in your decisions and the therapy. He's not a guinea pig, you should be the one "testing" and meeting the therapists first. Your husband should go as well. Wanting to fix this alone is not fair on your husband and the other members of your family.
If you have daughters: Your girls, if you have any, will despise you later if you decide to "fix" the violent kid and you always comply. Especially if they suffer from your neglect or from that violence or both, directly or by proxy. They will want to be nothing like you if you look weak — and complying to a 12yo may be perceived as weak especially if he uses violence and coercive ways. You are also teaching them what you think women should accept and that it's ok to be hit or suffer the violence of a loved one. If you have sons, they might also perceive this because that's the standard you are setting.
You said your son burst in anger even after you complied... Think of a similar situation as an exercise: How would you feel if your husband, that you absolutely love and admire, would let people insult him in your presence and even let you insult him, without much reaction or consequence? You would probably be upset at his lack of self-respect, you would want him to get mad, rise up and be strong. You'd probably feel like bullying him to see if he can actually be strong. When you comply to your son easily, he also sees an adult he can easily dominate but the issue is that adult is his protector too. This is scary for a kid, parents are supposed to be the "strongest and safest" pillars. If he can dominate you and your husband, you 2 are not looking very safe to him. Can you imagine how much anxiety your compliance and good intentions may actually be creating in your child? That's why you might want to see a therapist with your husband and see if you 2 have the right mindset. Do not exclude your husband. It's important to both be truthful to each others and united. You as a mother absolutely need to NOT deal with your son's discipline alone or have little mom/son secrets, this is a HUGE mistake.
No matter if your son has some biochemical issues, psychological or mental problem, or is a future narcissist or psychopath or simply borderline... he cannot be allowed to hurt others. At this age he's probably nothing of the above and there's hope for him but not with homemade solutions. Also, know that it's very difficult to learn to live with any of these issues above and dealing with people who suffer from this is pretty much a life of emotional slavery. It's totally normal to not understand or not know how to behave with them and how to protect yourself from their drama/attacks. Even as a parent, you need to shield yourself sometimes from the emotional guilt attacks that will be used on you. Even "good" kids do it.
I’m telling you all this because your son sounds terribly like my brother who was violent with me (daily), who was doing the tantrum you described and all the destruction of walls, objects and even animals, obsession with his social status/appearance. Very superficial, liar, planning traps, doing triangulation to separate people, can't tolerate losing, clumsy in sports, little patience, terribly insecure, no particular skill, no art, looked at himself in the mirror every time he could, etc. He manipulated everyone who could have offered me help and I ended up totally isolated and vulnerable, and even more abused.
This started when he was about 7yo and maybe your son too started earlier but with the other kids (or pets). Now they probably found a self-defense technique (eg. mutual support, fight, etc.) so he cannot dominate them and turned to you because it works. If you are absolutely certain it's recent then maybe something else triggered this (eg. abuse). No matter what, don't theorize about his actions and see a therapist. You need to get his condition and behaviors on paper somewhere. Your kid will soon learn he can use the system against others, including you. You think the tantrums are bad? Wait until he gets older and plan fake suicide attempts to guilt trip the family when he cannot cope with the horrible "mistakes" (with consequences) he did or when he lies to the the authority...
I cannot write too much but I can tell you it does not get better if let alone. That destroyed my life and put me in a cycle of abuse as adult. For my bro, it gives him even more power to abuse others once adult since he is successful and not bothered by empathy. He did not stop as adult, only the techniques changed and he used them on others as well including his own kids. It's heartbreaking.
I developed those traumatic issues into adulthood (PTSD, nightmares) and very bad social skills since I've been taught these violent coward men-child are allowed to hurt me if they emotionally "regret it". I do feel like I was a scapegoat and my mom let it happened by pitying my aggressor and by preventing my dad from knowing some of the bad stuff she knew, to protect my bro from consequences he should have dealt with.
My brother is perfectly fine now in his own sick criminal ways, wealthy, has a family, good job, big house, money, is respected. Looks like a normal adult to others. My parents made their choice and probably underestimated the negative effects.
I'm not worried at all about your son, I'm worried about you and your (any) other kids.
My bro is most likely a psychopath or narcissist but he was first a boy with self-control issues, anger problems and an overprotective mom. To avoid any of these issues in adulthood, they need to be fixed now in youth.