How can I add a damage region to a map to take away say 100 health a second?

in ZeroEditor there is an option box (alongside water, texture, height, foliage. ect.)

simply go to the left side of the screen and select "new region" and place it were ever you want.

now go to the right side of the screen, and edit the parameters there. You can actualy choose how much it does to each kind of unit (humans, droids, vehicles, animals)

You mistake.In BF1 dont have such a things.BF1 have only death region that when you step inside you die.You can't choose how much damage to get from AI when step inside.Maybe there is something like this in BF2,but in BF1 no.

If you want death region to add follow this:

Place one new region.In the right,place one name (for example Death1).When you hit enter the same name will apply in the properties section too.When you do this save your map and go to the luascript.Find this:

--AddDeathRegion("Sarlac01")

change it to look like this: (from the example the name will be Death1)

AddDeathRegion("Death1")

save the lua and munge.Thats all.

the bf1 zeroeditor doestn support it, and possibly neither the game engine at the point it was made, you may try this

Region("DamageRegion damagerate=60 personscale=1", 0)
{
Layer(5);
Position(-80.937225, 59.502384, 119.114571);
Rotation(0.923375, 0.000000, 0.383934, 0.000000);
Size(7.198578, 27.678169, 13.317889);
Name("TestRegion");
}

add that inside your maps .rgn file
i have not tested this but theres a chance it will work
as you can see, position is its cordinates, rotation and size etc. dmagerate is the damage per second i believe and person scale is if damage is amplified by the unit class(droids wouldnt be affected) just a guess.

The only possible damage regions that can exist are for water, and that's hardcoded.