This is intended as part of a series of tutorials on how to set up some of the various features which people might use in 2.55.
I will include screenshots as much as I can.
This tutorial is on how to set up a room where you take damage automatically unless you have a specific item.
Here's the minimum you will need:
A damage combo
An item
First, select your damage combo.
Under Triggers, Other find the box labelled Timer.
This will determine how often the combo damages the player.
Next under Effects, check the box ->ComboType Effects.
Finally, under States/Spawning; put the item number of the item that disables the damage under Req. Item.
Check the box Invert Item Req.
Place the combo somewhere in the room.
Exact location doesn't matter.
That's it! Now while you are in the room without the item, you will take damage however often the timer is set for.
Remember that ZC measures time in 1/60 of a second; so setting the timer to 60 will mean it will damage you once per second.