Make monsters chase Player when they spawn

[dufferton]

I'm trying to make a map where monsters will spawn in constantly and chase down the player.

How do I make it so monsters know where I am automatically and chase me without actually seeing me first?

[scifista42]

Vanilla/Boom: Force the player to shoot a switch in order to open his way out of the start area, the shot awakening all monsters in the map.
ZDoom: Use Thing_Hate on the monsters immediately after they spawn.