Leveraging Temporary Hit Points for Interesting Combat in D&D
I have an idea percolating that comes from the crossroads of two realizations about Dungeons and Dragons5th edition. The first concept is basic, but it is simply that NPCs don’t have to be built according to the same rules as PCs. The rules work the same way, but an NPC spellcaster, for instance, doesn’t need to be built as a Level X member of Class Y.
The other realization is that temporary hit points are kind of magical. What I mean by that is that the fact that they can’t build on themselves, but can keep renewing whenever a given circumstance is true, means that you can toughen something up considerably if you give it temporary hit points, but you don’t undermine the ability to actually defeat that thing, once you get past those temporary hit points.
Specific Example
The magic of temporary hit points really struck me when reading an adventure for Adventures in Middle-earth. That adventure has a stat block for a guard that gives them temporary hit points every round, until they sound an alarm. In other words, once they fulfill their purpose, they are less potent in the scene.
I thought this was a wonderful use of that set of rules. The guard demands more of the PCs attention because they represent a potential greater threat for what they can do. Once they can’t do that thing anymore, they are much easier to deal with, themselves, even if the evolving scene then becomes more complicated.
Fictional Combat
It’s a common trope in action-oriented fiction that some characters are more dangerous in a fight until the person fighting them “figures them out,” and then everything falls into place. While hit points are an abstraction, and may represent this to some degree, the mechanics of hit points don’t do a good job of telling that story.
That’s not to say it still doesn’t work to say that hit points are an abstraction of health, luck, and vigor in combat. It’s just that lumping all three of those together means it’s harder to figure out when you took the wind out of an opponent’s sails, or when they ran out of luck. When it comes to d20 level-based systems, I’m not a huge fan of trying to carve up hit points into discreet packages, but hit points are just a resource for how long an opponent stays in a fight.
Confidence
What all this led me to is potentially creating a trait for NPCs called confidence. Not every NPC should have this, just important NPCs for which combat should be more of a puzzle than a straight forward game of attrition.
Confidence works like this—so long as something is true in a fight, the character with confidence gains X number of temporary hit points. What that something is may be obvious, or it may take the PCs doing some investigation or using insight to determine.
Lesser confidence. If a given condition remains true, the character with this trait gains temporary hit points on their turn equal to their challenge rating.
Greater confidence. If a given condition remains true, the character with this trait gains temporary hit points equal to 3 x the number of opponents they are facing on their turn.
Examples
Characters with lesser confidence may be zealots who gain that benefit so long as their altar or idol remains intact. They might be troops that are so heartened by their commander that they gain the benefit so long as their commander takes the field with them. They may also be creatures that favor the darkness so heavily that they gain that bonus if there are no bright light sources in the area.
In the cases above, if the PCs destroyed the altar, killed or drove off the commander, or created a bright light source, the Lesser Confidence trait no longer triggers.
As far as Lesser Confidence goes, it’s not likely to be something that makes a creature invincible, but it is something that will make a large group of monsters take longer to whittle down, giving the weight of their numbers more time to wear at the PCs resources.
Characters with Greater Confidence may rely on a specific weapon for their fighting style, or they may be heartened by holding an item that it has taken them years to attain. They may be enamored of a given comrade in arms, or they may be exuberant if a ritual is under way.
In this case, taking away the weapon or item, removing the ally from the fight, or making the ritual impossible to complete will stop Greater Confidence from triggering.
Math
I haven’t had a chance to try out the math on any of this, and I couldn’t comment as to how the above traits would affect a character’s CR. To be honest, these are traits I would be more likely to tack onto an existing stat block to make a fight more dynamic, rather than something I would “build in” to the assumed capabilities of a new character.
The point isn’t so much that the PCs should “power through” the temporary hit points as much as they should figure out what is providing them and could remove they source of the confidence.
As Always
If you happen to use these ideas, and have some thoughts on how they worked, please let me know. If I could work them into something, I’ll be providing an update as well.
What Do I Know? 
Lesser Confidence probably needs a floor that is higher than 1. The use cases you suggest in the post would often include creatures with fractional CRs – guards and such – and even just rounding that up to 1 is so small a number that it probably won't add even one round of combat to the creature's staying power. I think you've got the core of a super strong idea here, though.Greater Confidence is also interesting, though I'm concerned that its significance is _largely_ static within a campaign. For the most part, campaigns have the same number of PCs at 1st level that they have at 20th, but 15 THP/round doesn't mean the same thing over that range.So again, I think you've got a great core idea, but I think getting to just the right number to be meaningful is hard. The narrative side here is super strong – the DM just needs to say every now and again that the character glances at [thing] and seems bolstered for PCs to get what's up.
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Yeah, I need to revisit this and get a handle on the math. It was more a matter of trying to get the idea into some kind of shape so I didn't lose the thread of it. I was hoping extra eyes would help me narrow down the specifics.
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