Workplace harrasment

So this is happening with from quite some time in my workplace I'm serving my notice period which is till May 26th 2024, Now all of sudden the my company is pressuring me that they will extend my notice period due to the ongoing project (not delayed by me), I'm stuck as the organisation which I'm going to join, I've provided them a fixed date that I'll be available from June 1st and my current organisation is now all of a sudden pressuring me to increase the notice period, and even giving me threats that they won't issue my experience certificate or even if they have to in case they would destroy my image and write all the negative points in my experience certificate. Please help me with any possible solution which I can opt to come out of this.