I have completed 70 days of my notice period, with 20 days remaining.

What you could have done:

• When you put down your papers, the same day you should have talked to your manager about your KT sessions and not taking up critical or lengthy tasks which might last beyond your notice period.

What can you do now:

• Arrange a meeting with your manager to discuss this scenario as soon as possible. The more you delay it, the more your manager has power to say that "you can't leave as your task is not completed" even after your official notice period ended. If possible, add HR as well in the same meeting.
• In the meeting, explain him the scenario, that this tasks might take longer that my notice period and you will not be able to complete it within notice period. You need another colleague whom you can teach or make him/her understand the tasks so even when you leave, they have someone who has knowledge about this task tland they can't stop you saying that you have dependency.
• Ask them to provide someone to whom you will transfer the knowledge about the things you worked upon till date within this project. Arrange regular KT sessions and record them and share recording links to the colleague who was involved in the KT sessions. Put your manager in the CC and put your personal email in BCC.
• Meanwhile try to negotiate a bit late joining date with your new firm as current firm might delay your relieving. You can convince your new firm by saying that you want to have some family time or function or anything that might convince them.

All the best!