I work for a very toxic organization. I am notice period but I fear that my manager will try to get it extended. I dont want to work here anymore and it almost feels like harrassment. What can I do here?

Be firm, as per your contract you are liable for only the number of days mentioned in the offer letter.

If they write over mail, or communicate verbally, tell them [date] is last working as per my contract and further time will be chargeable based on the new company’s wages.

Just keep things in writing and they cannot legally do this