Are there instances where a character can cast a non-cantrip spell while having no spell slots available and they later suffer negative consequences?

Question

Last night, I DMed a game with only two players, and neither of them had any prior experience with D&D. All in all, it went great, but towards the end, they got into some serious trouble and the druid seemed somewhat surprised by the fact that she had used all of her spell slots. I didn't want to punish a beginner too hard and the only alternatives appeared to be a near inevitable TPK or some cheesy deus ex machina, so I told her she could try to cast Healing Word despite having used all of her magic power for the day. I let her make a CON save to decide how she could handle the enormous stress of stretching her abilities to such an extend. She rolled quite high, but not extra-ordinarily high, so I decided that she could indeed successfully cast the spell, but that it might backfire later in some way. I haven't decided the specifics yet and in order to keep it interesting but balanced, I am looking for something similar to this in any official source book.

I'm aware that I'm well into homebrew territory with that ruling, and that this is not the right site to ask for inspiration. This is why I am specifically asking the following:

Is there any class or racial feature or any item that allows a character with no remaining spell slots to cast a spell of level 1 or higher at the cost of some negative consequence (e.g. taking a level of exhaustion)?

I am not asking for general ways to simply cast spells without expending a spell slot. There has to be some immediate trade-off. Taking 18 wizard levels in order to gain access to Spell Mastery can of course be seen as quite a trade-off, but I hope it is obvious that this is not what I am looking for.

Answer 1

Yes, but with caveats

This is after glancing through all published feats, races, classes, most items, and divine boons- though of course the sheer bulk of content means I very well may have missed something.

I also left out Sorcerer on a technicality, because although it can turn sorcery points into more spell slots, it's still using those spell slots to cast. That said, for practical purposes, the Sorcerer may be the best answer.

The best I can do so far is yes, through magic items, but with caveats: either the "cost" involved is weak and not (usually) immediate, or the cost will be significant but the spellcaster has to choose from an item/feature-specific list rather than their own spells.

Weak cost:

Rod of Absorption (DMG, p.195) allows the wielder to expend charges to cast spells from the Rod. The cost involved is that the Rod can hold a maximum number of charges over its lifetime, and so this will eventually destroy the Rod. However, it holds 50 spell levels worth of energy, so this "cost" is not immediate until the end of the Rod's lifetime.

(There are also many consumable items, which have a more immediate cost but also don't allow choice of spells.)

Limited spells:

This is probably the best I can find that adheres to the spirit of the question. There is the Eye of Vecna (DMG, p. 224), which can cast a variety of spells with a 5% chance per use that

Vecna tears your soul from your body, devours it, and then takes control of the body like a puppet.

Similarly, the Hand of Vecna (DMG, p. 224); whenever spells are cast from it, then

it casts the suggestion spell on you (save DC 18), demanding that you commit an evil act.

Runner Ups

Sorcerer

One of the main features of Flexible Casting (PHB, p.101) is the ability to transmute Sorcery Points into additional spell slots. Casting still requires a spell slot, but this is the answer that adheres to the spirit rather than the letter of the question.

Cast by Risk

There are other magic items that allow spells to be cast, either at some sort of risk to the caster or to the item. Examples are the Wind Fan (DMG, p.213) which has

has a cumulative 20 percent chance of not working and tearing into useless, nonmagical tatters.

when used more than once per day, or the Amulet of the Planes (DMG, p.150), which can cast plane shift but also has the risk of teleporting its user and companions to a random plane. Neither allow choice of spells.

4 Element Monk (PHB, p.80)

It's technically a spellcaster, and it can cast spells without spell slots at a cost (ki). This simultaneously technically meets the requirements of the question but also seems to deviate from the intent. This also falls under the "weak cost" category.

Shadow Sorcerer (XGE, p.51)

Has a feat that allows it to expend sorcery points to cast darkness directly; but again, weak cost and does not allow choice of spells. (Also, a sorcerer could have transmuted those sorcery points into a spell slot anyway.)