I have neurotoxin emitting in my level with the env fade set to fade out after 40 seconds to make it look like the toxin is filling the romm, but if you die the the env fade effect dissapears, the map goes back to normal. I want the effect to stay if you die, how do i do this.

The only way that I can think of is not to kill the player. Instead, immobilize the player, then change the camera angle to "simulate" death of the player.

This is sort of similar to the end of the official Portal 2 game, when the player was immobilized and had to shoot a portal to the moon.

How do you go about immobolizein the player

Try activating a game_ui with the freeze player flag set to on.