Blind Monsters

How do I make all monsters in a certain area blind? I want to include an area where the player is overlooking a large field with an army of demons but I don't want them all to attack right away when the player enters the cliff area.

one option is to put a big invisible wall in front of them. There also may be a way to do it with some self-referencing sector trickery, or by messing with the reject table

Yes, editing the reject table is a simple way to achieve this. link

You can even make it so you can walk among the monsters and they won't see you, only waking up when you leave their sector.

You can have all the monsters in a self-referencing sector and they'll only attack if you shoot them. It's a trick used in Requim MAP02, if I remember correctly.