Entities you need
trigger_hurt (using the block tool)
Making the deadly pit
Take your block tool and make a block, with the trigger texture, in the place where you want the player's death. After that press [Ctrl+T] and make it trigger_hurt. You can see many properties but we are only going to use the "damage", "damage cap" (only if you have chosen Doubling w/Forgiveness) and the "damage type" properties.
Final things you need to do
Change the properties
Property Name Value Help Damage 1000 The amount of damage done to entities that touch this trigger. The damage is done every half-second. Damage cap 1000 Maximum damage dealt per second. This field is only used if you select the Doubling w/Forgiveness damage model, via the spawnflag. Damage type FALL -