Anyone know of a good way to have a certain unit's health be slowly sapped while they are in a certain area? What I'm going for is having Marines slowly lose HP while they are in a certain area to represent exhaustion. When they enter another area, they will be revitalized. The main problem is, the modify health trigger is only for a percent. I thought of two approaches, but neither seem to be proving very fruitful.

1. Have 100 triggers per marine that go something like: if HP = 100& (a switch that is set), then wait 1000ms, set HP to 99%, clear the first switch, set the next one. Repeat down to 1% to 0%/kill unit.

2. Somehow set their status to permanently (it really doesn't matter if they're in an area or not) plagued. I know you can set units to invincible and hallucinated. I was wondering if there was a way to mod any of the editors to allow for that status. And I would need it to just affect those units, which is why I'm avoiding having defilers cast it. There are other units in the area that need to remain unplagued and cannot be made into units belonging to the plague casting player (in effect making them immune to it, if I remember correctly).

Anyone have ideas?

There is no "If HP =", excluding EUDs. You could have an invisible unit, say a Dark Templar follow and attack, but beyond having virtual HP, there's no way for you to increment or decrement hp without EUDs, which I'm not to knowledgeable about, so I'll wait for someone else who does know about those to come along.

It's quite possible to use EUDs to create an "if HP is less than/greater than/equal to" condition, IF:
You're OK with it only working on PCs
There is a fixed number of units you want this to apply to,
And those units must not be dynamically placed (either placed on the map or placed by a trigger that will always place them in the same order relative to all other units)

I could make an "If HP is equal to/less than/etc" condition using switches too. What I'm really after is something more like:
Conditions:
- Always
Actions:
- Subtract "1" HP for "all" "Marine" for Player "Player Controlled" at "Anywhere"
- Wait "1000" ms
- Preserve Trigger

Unfortunately, they only have a modify HP that sets it to some percentage.

If you really could, you would be my new hero. That's a big if though.
Could you explain to us/me how you can detect HP with switches?

If you don't want to make 100 triggers (which can be done quickly using a program) you could make a 'health bar' thingy. Check the wiki for it, I'm sure it's in there somewhere. Basically you create a lot of big units (buildings, mostly) on the edge of the map, on the minimap it shows a large line. When you give some buildings to a different player the line changes which can represent loss of health.
It's more complicated but it will require less triggers.

You cannot really do that because blizzard didn't include that option for the Set HP action.

Instead of using 100 switches you can use a death counter:
- If P8 deaths for Terran Marker (or any other unused unit) is exactly 100
Actions:
- Set HP to 99
- Set deaths for Terran Marker to 99
- Preserve trigger

And there's no way to set an unit status to plagued, except for placing an enemy defiler and making the computer cast the spell somehow.

Either you use vHP
or EUDs
or you use actual units that damage the affected units.
The plague sounds like a good option because all units are affected. You can make units that should not be affected invincible while the spell is being cast. Biggest problem I see here is to reliably force a cpu defiler to plague the units. But it can be done.

use vhp. EUDs would be too difficult and pointless. You'd need two triggers for each hitpoint.

It would go something like this...
Conditions:
- Switch "Start" is set
Actions:
- Wait 1000 ms
- Set unit HP to "99%"
- clear "Start"
- Set switch "99 HP"

Conditions:
- Switch "99 HP" is set
Actions:
- wait 1000 ms
- Set unit HP to "98%"
- clear "99 HP"
- set "98 HP"

...and so on.

What I'm thinking I might have to do is make use of the dark templar idea. Have one for each marine. Set a location around each marine and dark templar and then center them on the units. Then repeatedly send the dark templars after the marines on "attack location". Maybe even throw a few "move to location" in there so that the dark templars won't get side-tract on other units and buildings.

You're assuming that the units enter the area with 100% HP and won't take any normal damage while in the area. If you're OK with those conditions, then you don't need to do anything special at all. But I'd suggest using a death counter per-player rather than switches.

Another thing I'd suggest is lurkers buried around the area, or use other types of cloaked enemies. Depending on what you want this area to represent you could get different firing effects.

The dt method will probably not affect all units in the area by the same amount.

You can't detect HP with switches. You'd need EUDs.
What you do is just set the HP no matter what HP they had before. For example when a damaged marine enters your location he will be healed.

VHP seems the best way to do this if you don't want to use EUDs.
You could also put some cloaked ranged units around the area. The problem is that you'll the the firing sprites.

Why not make a separate timer? If you go into a hazardous area, why do you have to lose life, why can't you just die after you stay in too long? You can slow the unit down after they are in there too long as well.

Could someone link me to VHP? I can't seem to find it with the forum's search. Or is it in the wiki?

And as for death: It has to do with the map I'm making. I need the unit's health to go down so that this specific unit suffers attrition as well as damage from enemies.

Here is the wiki on it.

VHP

God damn you Falkoner.

I love you too rockz

Well, if VHP is not what you're after, scarabs are readily available damage sources, though that may not be the best idea by any means

cloaked wraith following the unit.

I'd nuke them

Any unit with splash damage might work. But it will not look as good because it wouldn't seem exhaustion.