Are there any broken applications of this homebrew "Overcast" system?

Question

I'm considering adding the following homebrew rule variant into an upcoming campaign:

Overcasting

If in dire need, any spellcaster is capable of expending their own life force to enhance their connection to the Weave. This allows them to cast one prepared spell without consuming a spell slot.

When you do so, you must make a DC 10 + Spell Level Constitution Saving Throw. On a failure, you take a number of levels of exhaustion equal to the spell's level immediately after the spell is cast. On a success, you take a number of levels of exhaustion equal to half of the spell's level, rounded up, immediately after the spell is cast. If the saving throw is failed by 5 or more, you take the levels of exhaustion immediately and the spell fails to cast.

You can only Overcast spells at a Spell Level less than or equal to your highest level spell slot plus one, the total of which is no higher than nine.

As items of this nature tend to have unintended consequences, I'm hoping to patch out as many exploits as possible before officially introducing it. What are some of the ways to grossly abuse this mechanic?

Answer 1

Elementals are immune to exhaustion.

Circle of the Moon druids can turn into elementals at level 10. Elementals have immunity to the effects of exhaustion.

The Beast Spells feature of Druids at level 18 allows casters to cast spells while in wild shape.

Example

Druid turns into a fire elemental. Being immune to the effects of exhaustion, the druid could cast extremely high level spells every single turn with no negative side effect. Before the druid leaves elemental form, an ally casts Greater Restoration on the druid, to lower the level of exhaustion from 6 to 5, so that the druid doesn't instantly die.

Answer 2

NPCs can use it

An NPC in combat with PCs is typically either going to win or die; goblin shamans, or ogre magi, or spellcasting evil dragons rarely get an encore appearance. Therefore, they have no disincentive for taking as many levels of exhaustion as they can cope with.

Answer 3

Even without druids, this is pretty broken

A single level of Exhaustion is really not that big a deal. One night of rest will cure it, it's why the Frenzied Berserker is not nearly as bad as people might think. But there's something far more insidious about this suggestion you're making, and that's that there's no such thing as an automatic failed saving throw.

Rolling a 1 on a saving throw does -nothing- that a 2 doesn't do, it's just 1 lower. (Except when it's a death saving throw.)

This means that anybody with who has a +10 on their constitution saving throw will never fail the check for level 1 spellslots. Anybody with +11 will never fail for level 2.

Now you might think, that's not that likely, but consider the following:

Let's make two broken wizards:

20 constitution Proficiency in constitution saving throws, from the Resilient feat

You're now already at +7 at level 1. At level 20, you're at + 11 already, and you've effectively got no chance to fail for spells of level 2.

Now let's get crazy:

• Resilient feat
• Level 20, for +6 proficiency (in Constitution saving throws)
• 20 Constitution for +5
• Luckstone for +1
• Cloak of Protection for +1
• Being near a paladin with 20 charisma and Aura of Protection for +5

Now you can't fail the saving throw, ever. So how do we abuse this? Well, simple, we have two Wizards with this build. Neither can fail their saving throw so neither will ever die instantly by casting a level 9 spell. (They can only gain 9/2 = 4.5, ~5 levels of Exhaustion per cast.)

• The first Wizard casts Wish to gain whatever effect he likes, gaining 5 levels of Exhaustion.
• The first Wizard casts Wish, to gain whatever effect he likes, he's now dead.
• The second Wizard casts Wish to replicate any resurrection spell to restore the first wizard back to life, gaining 5 levels of Exhaustion.
• The second Wizard casts Wish to gain whatever effect he likes, he's now dead.

Rinse, repeat, with the wizards switching position every time. You now have infinite access to spells of level 8 and below without requiring any of the material components, and all it takes is some mental scarring from casting yourself to death repeatedly.

You don't even truly need two wizards if you have somebody who can revive you and has enough components, but casually casting Wish over and over seems too flashy to pass up on.

Edit: An errata has clarified that being revived only removes one level of exhaustion:

Appendix A: Conditions[New] Exhaustion (p. 291). The fol-lowing sentence is appended to the last paragraph: “Also, being raised from the dead reduces a creature’s exhaustion level by 1.”

So instead of the Wizards reviving each-other with Wish, you'll most likely want a Wizard and a Celestial Warlock working together. As long as you have enough diamonds, the warlock can revive the Wizard 4 times per short rest, and the Wizard can keep killing themselves with the Wish spell.

Answer 4

In my evaluation these values seem pretty reasonable, even with some min-maxing, and not extremely abusable.

[EDIT] [Draco-demi-vanilla] liches (and other potential undead casters) rule the earth

They are immune to exhaustion, and potentially cast 9th level spells continuously and at will.

If BBEGs can do this, Legendary Resistance is more powerful

This means a lone caster that can cast 8th / 9th level spells can invoke an additional 9th level spell once every 5 days without risk of dying (and if they have greater restoration, with little consequence).

It makes Potions of Vitality (only Very Rare) amazing

Removing all levels of exhaustion lets you use Revivify to top yourself back off.

It allows a caster to build very tanky with less implications

For our hypothetical problem situation below, we'll be casting 9th level spells, e.g. Wish, Meteor Storm as much as possible.

It allows converting 7th-level slots into extra 9th-level slots

Per https://rpg.stackexchange.com/a/53555/38563, we assume that Resurrection removes all levels of exhaustion (though imposes a -4 to pretty much everything afterwards).

At the extreme, Resilient and 20 Con and +6 proficiency means +11 to saving throws, and 100 extra hp.

Lets see how many 9th level spells we can pull off before burning out:

Number of casts in pseudocode:

roll = d20 + mod
if roll <= 14: return 0  # A
if roll < 19: return 1  # B
# Have 5 levels of exhaustion, not dead (but disadvantage)
roll2 = min(d20, d20) + mod
if roll2 <= 14: return 1  # C
return 2  # D

In terms of probability, this is (for the ranges we care about)

p(0) = A
p(1) = B + C
p(2) = D

A = (14 - mod) / 20
B = 4 / 20
C = (1 - A - B) * (1 - (1-A)^2)
D = 1 - A - B - C

Approximate casts before death:

Mod |  0  |  1  |  2
----+-----+-----+----
+11 | 15% | 38% | 47%
+7  | 35% | 46% | 19%
+3  | 55% | 40% |  5%
-1  | 75% | 25% |  0% (0.3%)
-5  | 95% |  5% |  0%

Expected casts in the 5 lives before instafails (-6 or more)

9th level spells for free | Chance
--------------------------+--------
0                         |  2%
1                         | 10%
2                         | 24%
3                         | 30%
4                         | 22%
5                         |  9%
6                         |  2%
7                         |  0.2%
8                         | minute
9                         | vanishing
10                        | implausible

The average is 2.963625 (exactly 23709/8000, as it happens).

To run the above you would need a level 20 Cleric spending all day (4 hours) and energy (level 7/7/8/9 spell slots) reviving you, or 4 level 13s, or other equivalents, unless they wanted to roll the die themselves.

If you consider True Resurrection to remove this penalty (could not find any discussion on this) and you have no Wish penalties accruing then you can rinse and repeat.