Canadian employer issued ultimatum: commit to 1 year or else. Legal? If so, do I have any recourse?

I have been with the same employer for 3 years. To this point, my employment file is clean. I have received no verbal or written warnings, and am not on a PIP.

This morning I was issued a letter indicating that the company is aware that I have been interviewing outside of work hours. It accuses me of operating in bad faith, and being in breach of my employment agreement. I am not, and have confirmed as much. But, the conclusion of the letter issues an ultimatum whereby I must sign an employment agreement amendment committing me to 1 year of service. If I were to sign, vacating or terminating my employment within this time period would subject me to onerous financial consequences, and expose me to the potential for legal action.

Although it is not directly stated in the letter that I will be fired if I do not sign, this is clearly the intended message. My question is this: is this legal? If so, do I have any recourse? Granted, I am biased, but holding one's employment over their head seems to be the only instance of bad faith in these circumstances.