Create the trigger, group it with the player so you get the special options in the on activation box. Set it to repeatedly.
On Activation: poison=true; [] exec "poisonScript.sqs"
On deactivation: poison=false
In init.sqs write
poison=false
Name the player loon1. Now create your script file
; this script slowly poisons the player to death while he is in the trigger area
#loop
?not poison:exit
?getdammmage loon1 >=0.96:loon1 setdammage 0
loon1 setdammage ((getdammage loon1)+0.05)
~1
goto "loop"
Syntax not guaranteed - check it. This is not a working script, its just a sketch for you to use as a starting point.
You can change the parameters to suit yourself: with this script loon1 will die after about 20 seconds but you can make it as fast or slow as you like.