Is there i way i can make a thread invisible? I mean that the users wont be able to view the thread in the forum where the thread exists but if someone has the link will be able to view the thread!
You could put it in forum which has the display order set to '0'. It won't be visible by normal members but if you have the link, it will be shown. That is the only way that I could think of to possibly achieve this.
You'll have to find a workaround as the actual threads rows are shown in between <tr></tr>, meaning you'd have to style the table row or every element inside it. Maybe you could use a .children jQuery/JavaScript (not experienced in it) to add that value to it.