You have to beat null and then go to Daniilsuperx room, there are 2 hallways with a door, the door you chose will have a room with a code
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Step 1 - beat hide and seek and go to a red balloon that has a weird wall. If you walk a little bit you will find the null. Step 2 - beat the basic mode. Go to Baldi's office. Step 3 - go to Dani's library and find a red head. Click on the head. You will be teleported to a weird place and try to find the null in that place. Step 4 - done. Go to the menu and click on the play button. Go to the button that says others and click on null mode.