Low-HP 5e

Some quick ideas on how to lower the HP cap for PCs and NPCs. There's lots of discussion around why you'd want to do this, but some reasons you might consider:

• De-emphasizes combat.
• Emphasizes social solutions and creative problem solving.
• Encourages DM to come up with interesting interactions with NPCs.
• Up the realism.
• Monsters stay monstrous.
• Don't have to worry about balancing encounters.

Some relevant discussion here. I also swear I read a good article about capping HP in LoTFP or other such OSR recently but I can't find it for the life of me.

Note that while these changes should be applied to most NPCs and humanoids if they're being applied to the players, they should not be applied to monsters.

Constitution == Max HP.

Probably the most drastic change. A 1st-level character under this option is likely to be a little tougher than by default, but it's slow going from there. Only by increasing your Constitution score will your max HP increase, and even then it's a soft cap of 20 and a hard cap of 30.

I would probably limit characters to a single HD under this system.

This has the added benefit of being simple to apply to NPCs.

Ability Score Increase == HD.

Every four levels or so, you get another HD. Your max HP increases like the default rules.

This should produce gradual progress, and not require many other changes, but still have PCs with only about a fifth their default HP resources.

For NPCs you could probably just divide their HP and HD by 5 (round up).

Interesting side effect: High-level Fighters will end up with more HD than other classes.

Front-Loaded

You get HD and max HP normally until level 4 or so. Then you stop.

Pretty easy to eyeball this for NPCs. Since there's a cap you could even make a little table of Level/Con Modifier. Like this!

Con \ HD	1	2	3	4+
-2	2	5	7	10
-1	3	7	10	14
0	4	9	13	18
+1	5	11	16	22
+2	6	13	19	26
+3	7	15	22	30

You'll never have to track more than 30 points of damage on an NPC under this approach.

Small Max HP Per Level

Your max HP increases each level, but only by an amount fixed by your class, or your Constitution modifier, whichever is greater (minimum 1).

I'd probably set the per-class level based on a ranking of HD, so:

1. Sorcerer, Wizard
2. Bard, Cleric, Druid, Monk, Rogue, Warlock
3. Fighter, Paladin, Ranger
4. Barbarian

NPCs that use d8 for HD end up with about 40% of their default HP.

Magic Considerations

Under any of these, spells that grant HP increases like aid and heroes' feast become much more powerful. I would consider reducing any increases to max HP based on the spell level, or they'll likely become something the party feels like they always have to cast, and that's not interesting. Aid might grant 2 HP per level of casting instead of 5, for example.

Low-level healing spells remain useful for much longer, which means more high-level spell slots are available for other types of magic.

High-level destructive spells become one of the few hopes of defeating high-level monsters in direct combat, which feels right in a certain kind of game. Wizards and sorcerers become more like the glass cannons of older editions.