Since our session 0 we've included the Potion Miscibility variant rule but never used it 'til now.
The party (lvl 5 now) has hoarded quite a few potions – all common or uncommon level – and only used healing potions. At the end of the last session the alchemist decided they would mix a few together, after a few tries of varying success they rolled 100 on mixing a Potion of Growth and a Potion of Fire Breath.
The rules for rolling 100 on mixing potions is:
Only one potion works, but its effect is permanent. Choose the simplest effect to make permanent, or the one that seems the most fun. For example, a potion of healing might increase the drinker’s hit point maximum by 4, or oil of etherealness might permanently trap the user in the Ethereal Plane. At your discretion, an appropriate spell, such as dispel magic or remove curse, might end this lasting effect.
I ended the session with a dramatic description of their swirling potion – however I'm not really sure how to proceed. The players had a win and I want them to be rewarded for that – I'm just not sure which effect is most balanced. Also, unsure whether "Choose the simplest effect" means DM chooses or the player chooses – may not even be up to me- I've asked that in a separate question. Even if it is the player's choice, an idea of the imbalance this could cause would be useful to discuss with them/prepare myself for.
Permanent Growth: A player gets permanently enlarged – gains a size (medium->Large or small->medium), +1d4 damage to any weapon they were holding at the time and advantage on strength checks and saving throws.
Permanent Fire Breath: A player permanently gets a breath weapon that
A bonus action to exhale fire at a target within 30 feet of you. The target must make a DC 13 Dexterity saving throw, taking 4d6 fire damage on a failed save, or half as much damage on a successful one. The effect ends after you exhale the fire three times or when 1 hour has passed.
I considered making the breath weapon have no time limit, but still only 3 uses. However, that feels like it cheapens the win. I don't want to do that to my players.
As a comparison, they're both like getting a second level spell free…permanently. For fire breath the save is likely lower, but its damage is better than the 2nd level dragon's breath spell – though that is an area effect and fire breath is a single target. Growth is a straight copy of the 2nd level Enlarge without the option to reduce.
Which of these is the most balanced option?
Best Answer
Either effect is going to have repercussions.
Exact balance can be hard to determine
The two effects can be reduced to damage per round, but that's really not going to tell you much about how it's going to affect your characters in your game.
Basically, the character will be getting a magic item. In campaigns with a lot of magic items the addition probably isn't that significant. In a campaign where at level 5 there are no or few magic items, the effect will be greater.
The breath weapon is likely more powerful
For many characters the breath weapon is going to be more powerful, just because it does more damage. However, for a martial character, larger weapons doing an additional 1d4 is significant, and for many characters advantage on strength checks and saving throws would be, well, advantageous.
You need to deal with the issue of a breath weapon as a bonus action
Of significant issue is that the potion of fire breath says you can use a bonus action to breathe the fire. This is a significant feature, if permanent. Some DM discretion is called for here. A permanent breath weapon bonus action is significant, and as a DM I would pretty likely rule that out as a hard no, not happening.
Although, after looking at it, you may find it's not significantly overpowered even as a bonus action. Or perhaps the player is really keen on the bonus action feature. If you want to allow the bonus action but limit it, you could make it usable 3 times daily, which is reasonable based on the original potion. That way they'd get to have it as a bonus action, but there would be some limits.
Other than that, the breath weapon is not going to be outrageously unbalancing
In the end, depending on your game, the breath weapon is probably not going to be outrageously unbalancing (assuming you account for the bonus action), any more than many magic items. After all, the damage at an average of 14 on a failed save isn't extreme, especially since it doesn't scale with level, and depending on how you rule, it costs an action or bonus action that the character can't be otherwise using.
Take the player into account
To me, what the player wants is probably pretty important, and so you might consider discussing with them, so that you can figure out the most fun option.
Good luck!