I'll make a longer post when I care more, but:
Quick and dirty summary of Magnus' civil liability
(this is just me recalling my study material, since I'll have to memorize all this shit for the Bar and also I'm too lazy to get up and check my notes. Also different jurisdictions will have different approaches/standards to common law tort and contract liability. Again, this is just the bare bones.)
Battery (intentionally causing a harmful/offensive act to one's person causing damage.) Strongest claim bc Magnus literally stabbed Nathan. Magnus isn't making it out of this one alive (or at least not without significant fines).
Trespass to chattels/conversion (intentionally interfering with one's personal property and causing damage) Magnus' destruction of the apartment and all the stuff inside. Tacked conversion on too because some of the items Magnus' destroyed could have been fucked up so bad that he'd have to pay back the full market price (e.g. if the guitar he broke was limited edition or had some other unique quality).
I'm having trouble classifying Magnus' "revenge is coming" threat under tort law. It's not imminent enough to be assault (like if Nathan saw him say it as he was raising his knife), and it didn't seem to cause the reaction required for an intentional infliction of emotional distress claim. The property damage from the message could come up as a tort, but not the message itself. In a criminal case, it could be used as evidence of mental state for...all of Magnus' future actions (but I'm stopping myself before I start bringing evidence law into this, because there would be so many hurdles the message would have to clear and I'm already using my property law studying day to ramble about applying tort law to my favorite cartoon)
Magnus' claims (none are good but he can try)
Battery. Nathan did punch Magnus hard and repeatedly enough to cause permanent damage to his eye. However, Magnus was the aggressor. Magnus' counsel could argue that Nathan's retaliation was not proportionate because once Magnus was knocked to the ground and let go of the knife, the threat was gone. Plaintiff's counsel would just have to say "he stabbed him in the shoulder" and Magnus' counsel would consider quitting right there. (who are we kidding: if this goes to trial, Magnus is doing this pro se)
Defamation: Nathan called Magnus crazy. This is getting thrown out. (no harm to Magnus' reputation bc he doesn't have a reputation to harm)
Nuisance: Pickles was playing the wrong drum pattern. Judge is probably dismissing the entire case for that. (if the neighbors didn't raise a nuisance claim against them already, it's not happening now)
Anyway, here's my rough estimation of Magnus' damages with the band
Nathan's medical expenses
Repairing/replacing stuff in the apartment
Lost profits from the band not being able to use their instruments or not being able to work in general
Not including the remedies for the breach of contract claim with Crystal Mountain (I'd love to analyze that too, but idk what the contract looks like).
My headcanon for how this would proceed: Magnus knows he can't afford this shit, Crystal Mountain knows Magnus knows he can't afford this shit. Crystal Mountain has lawyers, and the band has Charles. Magnus has nothing but a weak case that will make any judge and jury view him as a violent weirdo. Magnus is going to have to settle out of court. In exchange for either reducing damages or eliminating them altogether, Magnus has to sign away all of his rights to everything Dethklok-related and essentially write himself out of their history. The end. Alexa, play The Hammer (soft piano cover).