Hello Anonymoususer2013,
Like a few other members suggested--now-a-days, every Offer Letter, especially in the IT sector & MNCs contain the Background Clause & the consequences of unfavorable BC results. Many companies also mention it in the Interview Process.
Ignorance of the process is no reason for a unfavorable view of the consequence(s).
Coming to the facts of the case that you mentioned.
1] You say that this PM is NOT your Reporting PM. Then where was the need to go all-out against him, in the first place? There do seem to be other facts of the scenario that you have missed/omitted to mention.
IF you wanted to be a whistleblower, nothing wrong in it per-se. BUT I think you forgot to take into account the consequences. Just watch the TV for a couple of days & you will know what this entails--that's part of the game for any & every whistleblower. In a single line: Life IS NOT a bed of roses for any whistleblower.
2] Coming to your remarks about what the PM has done/said is right or wrong, etc--pl note that what the other person [in this case it's your PM] says or does IS NOT IN YOUR HANDS/CONTROL. What's in YOUR control is YOUR RESPONSE to it. Suggest focus on that aspect of your nature. The very fact that the PM has gone to such length as to mention that he will ruin your career says a lot about the quality/type of interactions you had with him. With a simple visualization, I presume your PM--to whom you are reporting--WOULD HAVE suggested you to keep away from this guy.
IF you assume that I am suggesting you should have just ignored whatever you mentioned this other PM was doing--YOU ARE WRONG. There are multiple ways in which one can RESPOND--including resigning coolly without raising any stink, UNTIL the timing is right--since THAT'S WHAT is in your hands.
As regards going the Legal way, I am not sure what would you be achieving by taking that route--except feeding the advocate for a few years AND spending a few years of your time in pursuing the case. And in the end, there's NO GUARANTEE that you will get what you wanted when you began the case. It's YOUR priority that decides what you do--this would seem to be a case of thinking thru THE HEART when you ought to be THINKING THRU THE HEAD.
3] Now how to handle your new job situation. Dolphy has given an excellent suggestion--just meet the HR of the new company & explain the facts of the case. In fact, you should have made it a part of your interview process itself--every recruiter asks the reasons for changing the current job--WITHOUT sounding antagonistic towards the PM or the present Company, you COULD have given the actual reason. Most recruiters understand that such situations do occur & IF they are convinced that you are speaking the truth, they will devise ways to hire you--presuming your technical rounds have gone off very well [Obviously, if your Interview Performance has been just average, the recruiter may not find it worth his/her effort to help you].
In case this new job doesn't get revived, suggest look for other jobs--and make it a point to BRIEFLY mention the REAL reasons to change during the interview process.
All the Best.
Rgds,
TS