A few points here......
Was this person actually working for the company when you wrote this? If they were then you could certainly be disciplined for it, it would be seen as a form of bullying I'd guess. If they dont work there anymore, and weren't when you wrote it, then you should be ok (as far as action from the company is concerned).
If, as you say, it was a 'character assassination' on the person then I doubt if you could 'rip it shreds in a court of law any day of the week'. Personally, I wouldnt be so arrogant in this situation.
As far as the company is concerned it will come down to whether the person youve insulted was actually working for the company or not. Whatever happens though, it wont have put you in a very good light with your employers as a complaint has been made.
Reading your original post again, as they have raised a grievance I assume they were employed by the company at the time otherwise they couldnt raise a grievance. So, yes, the company should take this very seriously and investigate it. It will depend on what exactly you said about the individual as to any action they take.