Map authors who want to prevent players from reaching certain areas can use kill volumes (aka death barriers or kill zones). These are cuboid 3D trigger volumes which are scripted to instantly kill any player that enters them. Adding them to a map is easy:

Adding trigger volumes

The first step is to add trigger volumes to your map's scenario using Sapien.

1. Select "Trigger volumes" in the Hierarchy view
2. Right click on the ground in the Game window to place a volume
3. Left click and drag the faces of the new volume to adjust its size
4. Rotate the volume using the active marker if needed
5. Using the Properties palette, give the volume a suitable name like kill_volume (should be lower-case)

To cover a larger area, you may need to place multiple volumes with different names.

Writing the kill script

Now that the volume(s) are placed, you'll need to write a script to continuously test if players are within this volume, and kill them if so.

Supposing your level was Danger Canyon, you would place the script file at data\levels\test\dangercanyon\scripts\kill.hsc. You may need to create the scripts directory for your level if it didn't already exist. The name of the script file, kill.hsc, is not important other than it must have the .hsc extension.

In the script file, paste the following:

(script continuous kill_players_in_zone
  (if (volume_test_object kill_volume (list_get (players) 0))
    (unit_kill (unit (list_get (players) 0)))
  )
  (if (volume_test_object kill_volume (list_get (players) 1))
    (unit_kill (unit (list_get (players) 1)))
  )
  (if (volume_test_object kill_volume (list_get (players) 2))
    (unit_kill (unit (list_get (players) 2)))
  )
  (if (volume_test_object kill_volume (list_get (players) 3))
    (unit_kill (unit (list_get (players) 3)))
  )
  (if (volume_test_object kill_volume (list_get (players) 4))
    (unit_kill (unit (list_get (players) 4)))
  )
  (if (volume_test_object kill_volume (list_get (players) 5))
    (unit_kill (unit (list_get (players) 5)))
  )
  (if (volume_test_object kill_volume (list_get (players) 6))
    (unit_kill (unit (list_get (players) 6)))
  )
  (if (volume_test_object kill_volume (list_get (players) 7))
    (unit_kill (unit (list_get (players) 7)))
  )
  (if (volume_test_object kill_volume (list_get (players) 8))
    (unit_kill (unit (list_get (players) 8)))
  )
  (if (volume_test_object kill_volume (list_get (players) 9))
    (unit_kill (unit (list_get (players) 9)))
  )
  (if (volume_test_object kill_volume (list_get (players) 10))
    (unit_kill (unit (list_get (players) 10)))
  )
  (if (volume_test_object kill_volume (list_get (players) 11))
    (unit_kill (unit (list_get (players) 11)))
  )
  (if (volume_test_object kill_volume (list_get (players) 12))
    (unit_kill (unit (list_get (players) 12)))
  )
  (if (volume_test_object kill_volume (list_get (players) 13))
    (unit_kill (unit (list_get (players) 13)))
  )
  (if (volume_test_object kill_volume (list_get (players) 14))
    (unit_kill (unit (list_get (players) 14)))
  )
  (if (volume_test_object kill_volume (list_get (players) 15))
    (unit_kill (unit (list_get (players) 15)))
  )
)

Because Halo scripts do not support loops, we must repeat the test for all 16 players (indexes 0 to 15). For a singleplayer map, just testing for players 0 and 1 is sufficient since MCC only supports up to 2 players in co-op. Note that the script contains the name of the trigger volume, kill_volume, so adjust if your volume is named differently.

If you created multiple trigger volumes, you will need to repeat the above code for each trigger volume name.

Compiling the script and saving the scenario

The final steps takes place back in Sapien. From the File menu, select "Compile scripts". This will convert the raw script source from the data\levels\...\scripts\ directory into compiled script data in your scenario. You should see "scripts successfully compiled" in the Game window. Save the scenario since you've added scripts and trigger volume(s).

At this time, you should compile the map and test the kill volumes ingame.