[RPG] Allowing more spell slots at the expense of fatigue: Will this house rule break the game

balancednd-5ehouse-rules

I'm playing D&D 5e. PHB (Chapter 10: Spellcasting) states that (emphasis mine):

Regardless of how many spells a caster knows or prepares, he or she
can cast only a limited number of spells before resting. Manipulating
the fabric of magic and channeling its energy into even a simple spell
is physically and mentally taxing, and higher-level spells are even
more so. Thus, each spellcasting class's description (except that of
the warlock) includes a table showing how many spell slots of each
spell level a character can use at each character level.

I want to emphasize this aspect in my homebrew world so I have come up with the following house rule:

Inner Reserves: A spellcaster can exceed the number of spell slots normally allocated to her at the expense of getting fatigued.

When a spellcaster runs out of spell slots she can access her inner
reserves and push her physical and mental abilities to their limits.
Doing so, allows her to cast a spell but gain an exhaustion level
equal to the level of the spell she cast. For example, if the
spellcaster casts a Level-2 spell she will gain two levels of
exhaustion.

The exhaustion takes effect at the end of the spellcaster's turn.

Reasoning

I would like to use this house rule for several reasons:

  • Fair Exchange: In my eyes, this is a fair exchange. Gaining one level of exhaustion will most likely not make a difference in a battle but that is also the case for a Level-1 spell. On the contrary, when gaining two levels of exhaustion or more, the PC becomes extremely vulnerable. So this is a high-risk high-reward scenario. If you are about to exhaust yourself you better make sure that you will win the fight.
  • Dramatic Effect: I think it goes without saying that such a mechanism can lead to some very dramatic moments. I can imagine several scenarios where a spellcaster exhausts herself to heal the group or cast one last fireball that ultimately saves the day.

Potential Problems

An immediate problem I identify is that this rule allows all classes to cast 5 more Level-1 spells (or less at higher levels). This may break some classes that are designed to cast very few spells per day (e.g., Warlock, Paladin). On the other hand, I feel that this imbalance is mitigated by the severe consequences of high exhaustion levels. I.e., I don't think that it's viable to abuse this mechanism.

Questions

  1. Will this house rule completely break the game?
  2. Which classes are going to be affected the most?
  3. Are there any other unforeseen consequences introduced by this rule? (i.e., unrelated to classes)

Finally, if this rule does not completely break the game, I would ideally like to read about how you'd improve it, but I feel that this may push rpg.stackexhange.com's rules a bit, so feel free to omit this part.

EDIT: @encryptor's answer includes some excellent points. I have edited the rule's description to address some of them.

Best Answer

This will not "break" the game; for now

There is no way such a rule "completely breaks the game", until the party finds a reliable method of removing exhaustion. After that, spellcasters become over-efficient. Using Potion of Vitality in sufficient quantities can be such a method:

When you drink this potion, it removes any exhaustion you are suffering

The rule effectively gives extra spell slots, thus makes the game a bit easier. DMG suggests even more serious changes which affect spellcasting, like 1-hour long rests.

This will definitely change your game though, especially for lower levels. It will become more useless as the spellcasters progress, but in the end such a house rule — changes in the game mechanics for couple of heroic moments — is unnecessary.

For the reasoning you've made a ruling will be enough

Rules have costs. When you implement a new rule, you have to work it out, you have to write it down and edit it for clarify, you have to present it to the table and ensure everyone’s understanding of the rule, and then you have to devote game time to enforcing it.

I think it goes without saying that such a mechanism can lead to some very dramatic moments.

To achieve this, you don't need a house rule. Make a ruling. When the moment comes, tell the player "on the last leg, you can cast an additional spell for two level of exhaustion; will you do that?" This decision have a couple of advantages:

  • It is fair. You named the price, you gave an opportunity, but the player is still in control. As a DM, you can vary the price based on circumstances, to ensure it will always be appropriate.
  • It is dramatic. Before that very moment the player didn't even know this would be possible and was going to die, when suddenly - bam! - here's the last resort. On the contrary, knowing an extra spell slot "safety bag" is always available makes the situation less tense.
  • It can't be exploited. As an ad-hoc decision, it leaves the game mechanics intact.
  • Players didn't have to learn new rules, they can pay more attention to their characters instead.
  • It portrays you as a good GM, that always comes with a creative decision when saving the party. Others just fudge dice, go with "you were captured" pabulum or allow TPK.

On the other hand, a specific house rule has to be universal and error-proof, thus requiring much more attention and effort from the players and DM alike. This particular rule has a few problems:

  • It gives the same number of slots for any level, any class. It does not scale at all. Low-level casters make much more of it.
  • Different classes have different spell slot numbers. It is more valuable for classes with fewer number of spell slots like Ranger, but almost useless for, say, a high-level Wizard.
  • Sorcerers can make more use of it because of Font of Magic
  • Characters without spell slots make nothing of it, although they still might cast spells from feats or class features.
  • Being a "panic button" it might not come into play at all, so its value becomes zero, while its cost still remains.
  • Alternatively, players might turn it into a pure resource-management tool, which I believe wasn't the intent.
  • The trigger "when a spellcaster has no more spell slots but deems it absolutely necessary to cast a spell" is ambiguous and vague. How necessary is "absolutely necessary", who decides it? Also, if I have one 1-level spell slot left, I still can't use this rule to cast Fireball, despite of the example.

Don't change game mechanics, introduce a homebrew trait instead

Here's a mild version of the rule I want to suggest:

You can cast a spell you are normally able to cast, expending exhaustion levels instead of its normal cost. The level of the spell becomes its cost in exhaustion levels (minimum 1). You can cast a spell once this way and regain the ability to do so when you finish a long rest.

It should be a feat, or a racial trait, which players are able to choose.

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