Quote from TeamSpen210 on July 18, 2014, 10:48 pm

For particles, you'll need an info_particle_system. Set it to "human_cleanser" for the neurotoxin cloud. I'd suggest you extract the "particles" folder from the portal 2 vpk with GCFscape, then put it in "portal 2/portal2/" so it appears in the particle browser. There's a neurotoxin vent overlay you can put on wall tiles, or a texture version as a panel. Put the particle in the middle with the "cone" arrow facing away from the wall so it looks nice.

Surround the map with a trigger_hurt, set to start disabled with a high number for damage. It's fine if it sticks out into the void, as long as the center/origin is inside the map. You'll also want an ambient_generic with the neurotoxin sound. In the flags "play everywhere" and "start silent" should be checked.

For the timer display, you want a vgui_neurotoxin_countdown. Put it 1 unit in front of the screen of a glados_screenborder monitor or something similar, and set the width and height to match the size. In the properties it has an option to set the time in seconds.

To start the neurotoxin, Start the particle_system (name them all the same to make it easier), PlaySound the ambient_generic and Enable the vgui display. After the time has elapsed, Enable the trigger_hurt so the player dies.

When the player has won/stopped the nueurotoxin has stopped, Stop the particles, StopSound the ambient_generic, Disable the vgui_display and Kill the trigger_hurt so it can never activate again.

If you need examples, try decompiling sp_a2_bts1 - there's two different neurotoxin traps, one in the darkened test chamber (if you don't escape with Wheatley GLaDOS will try to kill you) and the other trapped test chamber GLaDOS tries to lure you into. Valve has various covers on their versions in the tests which open to reveal the screens, but that's not difficult to make.