Didn’t join the company after accepting Offer Letter and company has sent me a legal letter

Greeting, I have signed a MoU and Bond with a Company when accepting their Offer Letter in which its mentioned I will work here for 2 year. Also it was mentioned that neither you look for another job nor company will look for another candidate. if anyone break rule, they will pay 1 month salary. To be honest, the company has waited 3 months for me. I got to know that the company has very long hours working culture and I got better opportunity. So I have informed the company via email at the end of 3rd month. And I joined another company. Now I received a legal notice from their lawyer about the same thing that I have pay 1 month of salary. Also they have mentioned that they will include my new company in this matter if needed and tell my new company about breach of MoU and Bond.. I really found this thing very cheap and unprofessional that they are dragging my current company into this matter even they have nothing to do with this. I more feel it as threatening. Note: I have not even taken single penny from that company. I didn’t join the company for 1 hour even.