In my last campaign, some combat encounters got pretty ridiculous, mainly due to some player characters falling unconscious and getting back up every turn. I remember one combat encounter in particular, during which the cleric of the group had his turn right before the rest of the party in initiative order. They got knocked down by the enemies, and then the cleric brought them back to consciousness with a mass healing word. They stood up, had their turn and were reduced to 0 hp again. This happened like 4 or 5 times in a row.
The players were glad that they survived this encounter, but agreed with me, that this little procedure of losing consciousness, then bouncing back, over and over again, is kinda stupid. So, I envisioned a homebrew rule to deal with this situation:
After regaining consciousness, the character would roll a Constitution save with DC 10. A successful save would result in no bad consequences, but the next time this happens, the DC would be increased by 5. On a failed save, the character would gain a level of exhaustion and the DC for the next time this happens would be back to 10. After a long rest, the DC is set back to 10 as well.
This way, there is an inherent downside to repeatedly falling unconscious, but it’s not immediately crippling for the character. (Plus, this house rule would be applied to enemies as well, unless they are undead or constructs).
Note: In my campaign, NPCs make death saves as well. The number of death saves they are allowed to fail depends on their importance to the campaign. Named NPCs (regardless of whether they are allies or enemies) usually die after two failed death saves, whereas unnamed NPC die immediately or after one failed death save, depending on the situation. This allows NPCs to roll a Natural 20 and rise with 1 hp, which has led to some really awesome moments in my campaigns.
Now my questions are:
- Will this houserule unbalance my game?
- Do you see any likely troubles that will result from this?
- Does this unfairly favour/disadvantage any characters (classes, races, archetypes)?
This house rule will almost certainly change the balance of your game -- by making it more lethal -- but whether it will "unbalance" your game is for you and your players to decide together.
Imposing save-versus-exhaustion rolls when PCs regain consciousness after dropping in combat creates a nontrivial risk of a so-called "death spiral." Once your players start losing, they'll lose faster and harder.
The first two levels of exhaustion will make combat modestly more difficult, but at the third level of exhaustion, characters suffer:
PHB p. 291 (emphasis mine). Once that happens, your PCs will suddenly find themselves failing spell saves they would otherwise make; taking damage they otherwise wouldn't take, and so falling unconscious more often; failing death saves they would otherwise make; and failing further saves versus exhaustion if they manage to regain consciousness before dying. Remember that death saves are already challenging, because with rare exceptions they are flat rolls with no modifiers for, e.g., high attributes or proficiency. Making death saves with disadvantage will make double-fails on rolled 1s more likely, and will make it dramatically less likely for anyone to roll that clutch 20 and spring back up with 1 hit point. (See PHB p. 197.)
And just to drive the point home, I'll note that a PC's hit point maximum is halved at four levels of exhaustion, and at six, the PC straight-up dies. No save, no second chance. Dead.
Moreover, at least with respect to published adventures, you might find that PCs will already be suffering from exhaustion from various sources at higher levels of play. I recall my own experience going from Tier 1 to Tier 2 in Adventurers' League play and discovering that I spent roughly every other session with at least one level of exhaustion. Playing a monk, a class that tends to rely heavily on ability checks (for things like Acrobatics), and constantly having disadvantage on all ability checks was a drag. (And that's to say nothing of playing a mobility-focused character who often suffered from half-speed at exhaustion level two. Ugh.) Had I to contend with saves versus exhaustion every time I dropped in combat, I expect that character wouldn't have survived.
Further balance changes to consider: your house rule makes Constitution more important than it already is. Players wanting to avoid a death spiral situation might make different character-building choices as a result, so as to prioritize Constitution more highly. Be prepared for a party full of barbarians. Likewise, anything that grants a bonus to saving throws -- such as the monk's Diamond Soul ability (PHB p. 79), the bless spell (PHB p. 219), or the stone of good luck (DMG p. 205) -- becomes more valuable.
Finally, the other part of that third level of exhaustion -- disadvantage on attack rolls -- arguably affects non-caster classes disproportionately. Disadvantage on saves will of course make it more difficult for casters to maintain concentration-based spells. (See PHB p. 203.) However, a wizard can at least sling non-concentration, save-based spells (e.g., fireball) to full effect despite exhaustion. Martial classes whose efficacy is heavily dependent on connecting with attacks will have it rough when all of their attack rolls are made with disadvantage.
All that said, the relative lethality of a game is a playstyle choice like any other. Some groups enjoy a gritty game where death lurks 'round every corner. If your group digs that kind of play, and you all will be heading into a more lethal game with your eyes open to the possibility that it'll be a short one, then there's no reason not to do so.