How should I handle giving a certain player a special ability in order to encourage roleplay?

Question

I have a tiefling bard in one of my campaigns. In the campaign's universe, there is a lot of prejudice against tieflings.

I wanted to encourage some roleplay by adding a homebrew rule that, when she makes a spell attack roll, she also rolls a d100 with a DC of 100. If she fails, the DC is lowered by 1. If the roll is successful, she loses control of her magical abilities or emotions and she casts fireball on accident; this causes any NPCs with prejudice to fear her more, and certain specifics such as tavernkeepers will not let the party stay at their tavern.

How can I balance this and not make the PC overpowered or make it seem like I'm favoring her?

Answer 1

Step 1: prepare answers to "obvious" questions.

Note that these aren't questions that we need answered in order to answer your question, but that probably need to be answered within your game world.

Note also that the answers to these questions may be "I don't know yet" - or, potentially more interestingly, "the characters don't know yet", and may lead to great arcs as the party tries to grapple with the issue or resolve complications it may bring.

Question 1: is this thematically appropriate?

Why is this ability appropriate for this character? Are they generally hot-tempered or are they usually calm?

Is this a racial feature? If so, why is it thematically appropriate for tieflings, generally?

Question 2: why fireball?

Has the character chosen fire-themed spells or otherwise leaned into a fiery motif? Do tieflings in your world generally have a fire motif?

Question 3: can the player influence the roll?

If this keys off of emotional state, is there anything that the player can do to prevent - or encourage - a fireball "mishap"?

Can they use advantage (eg., from Inspiration) to roll twice? If so, can they choose to take the lower if they really don't want fireball going off?

Can they use other floating bonuses (eg., Guidance) to affect the roll?

Can they take an action of some sort to center themselves to avoid (or intentionally trigger) a fireball? Previous editions have had full-round actions, which take both your action and your movement for the round; could the bard trade their movement (and/or bonus action, I suppose) to nudge the roll in their favor?

Question 4: why does this happen?

You suggested that this is a loss of control of "magical abilities or emotions". Which is it? How can the character learn to keep control? Why can't this character (or tieflings in general) keep control enough to cast like every other caster in the world?

Question 5: does this scale with the spell slot used?

At first level, getting 8d6 surprise damage is great against goblins. At 15th level, or if triggered by an up-cast spell, that could be quite disappointing.

Step 2: get buy-in from the player.

This feels like a major change to the character's abilities; such changes should generally get buy-in from the player. Without that buy-in, the player may feel frustration and/or a loss of agency, possibly enough to want to play a different character or even leave the group.

Story time: I was in a short-lived 3.5e campaign in which this happened to a player who chose a minotaur (which was otherwise appropriate for the campaign); they accepted that minotaurs were "second class citizens" in most of the world, and that theirs had been recently freed from slavery (the details were murky about exactly how that happened). The GM then decided that the minotaur's horns would have been removed by their last owner (thus also removing their gore attack). The player was ... unimpressed, to say the least. In that particular case, the GM was full of other failings, so it's not surprising that the campaign fell apart after just a few sessions, but the horn issue didn't help anything.

And, to be frank, this player would almost certainly bail from the character - and maybe the campaign - if this change were forced.

Step 3: get buy-in from the rest of the players.

This feels like a big change to the character's abilities, and one that the other characters should become aware of fairly quickly (especially if it's a racial thing, not just this one tiefling). Why would a band of adventurers accept a team member that not only makes some NPCs more difficult to work with but also has a chance of accidentally frying them in combat?

IME, most players will bend over backwards to come up with an in-universe justification for their characters doing pretty much anything that forwards the campaign, from working with "undesirables" to parlaying with a pit fiend. But, each player has a limit. Talk to the players to make sure that this doesn't cross any of their limits.

FWIW, this player would put "fixing the bard" at the top of the priority list.

Step 4: don't worry about balance too much.

Skimming through the bard spell list again, there aren't a huge number of bard spells that require a spell attack roll. My suspicion is that, unless the player is trying to trigger this ability, it'll only come up once or twice in the whole campaign.

Most of my experience is in 3.5/PF, but the 5e campaigns I've been in feel close enough that I think this is relevant:

In my experience, one session will average 2-3 "major" combat encounters (that is, big enough to bring out the map and the minis - and I love me a good map and some minis). A combat encounter averages something like 4 rounds, so that's 8-12 rounds of combat per session. The longest single campaign I've been a part of lasted about 5 years, meeting twice a month on average (we had a player with an obnoxious work schedule, which didn't help that). So, 26 sessions a year - call it 30 for easy math. 30 sessions a year * 5 years = 150 sessions or 1,500 combat rounds. Ballpark, casters cast in, say, 2/3 of those rounds, so about 1,000 spells cast per caster; maybe half of which are AoEs. So, down to about 500 targeted spells cast.

If a short rest resets the chance of casting fireball, that will probably more-or-less counter the increased chances that come with casting more spells - at least, for this kind of back-of-the-envelope calculation. Given that, roughly 1% of targeted spells will spontaneously become fireball - that's about 5 castings over a 5-year-long campaign, very roughly.

If a long rest resets the chance, but a short rest doesn't, I'd about triple the chances and ballpark maybe 15-20 castings over that same 5-year campaign.

The 5e bard spells I've scanned through would reduce those chances a fair bit: lots of AoEs or "choose a creature" without a spell attack roll.

This GM would expect the bard to accidentally pop a fireball maybe once or twice over the course of a campaign. Honestly, getting a free wish once or twice over the course of a campaign probably won't affect game balance meaningfully.

A suggested adjustment to wording.

This GM would suggest wording the ability as:

Whenever they cast a spell [using a spell slot?] that requires a spell attack roll, Tieflings have a cumulative 1% chance of accidentally casting Fireball instead of they spell they chose. The fireball is centered on the creature or point that the original spell targeted. The chance of mis-casting resets when a spell is mis-cast or after a short or long rest.

Using terms like "DC" and "success" or "failure" suggests that this is something that the character/player has some control over or may want to have happen. As a player, I would rarely want this to happen; for me, succeeding would almost always be rolling a "failure".

Answer 2

Mechanic-focused and roleplay-focused are often at odds

Combat is generally the most mechanical and least roleplay focused part of a D&D game in my experience. As a result, fiddling with combat mechanics for a character is a relatively ineffective way to encourage roleplaying. A player who already wants to RP heavily will probably do so either way, but a character who isn't as motivated to RP may not be persuaded to do so via combat changes.

So in that sense I don't think that the proposed homebrew rule is likely to be effective in promoting roleplay, and so the easiest way to handle it is to not use it. Especially since it may not have much effect on roleplaying the outcome has a good chance of seeming like favoritism (if the PC gets a net benefit from it) or the opposite (if the PC gets net negative consequences from it). Those are very hard to balance across a table of players, most of whom don't have the same in-game rationale for any similar changes.

Focusing on narrative effects to encourage roleplay may help

Emphasizing the narrative effect of what you want is relatively easy to do as the DM, given your degree of control over the game and its denizens. The major thing that jumped out to me in your description is that this situation is more about other people's opinions about the Tiefling than the Tiefling doing things to justify or enhance those opinions. Someone with a strong prejudice might not allow a Tiefling to stay in their inn just because of that prejudice.

For mechanical representations of this, roleplaying elements can be about having the PC deal with those sentiments through more or harder Charisma skill checks. I do this a lot for modifying difficulty on quests when things like prejudice are in play: the "default" response of an NPC is to make things harder for a PC, or at least not help, and it's much harder to secure any assistance they might be able to offer. It works well because players recognize that something helpful might be available but is hard to access given the NPCs' attitudes, but doesn't bring play to a halt if they can't get that help. Losing access to an inn, and therefore a safe place to sleep, can be a good example of this.

But for the most part I try to keep roleplaying elements like this non-mechanical. Dialogue can be flavored in ways that make it clear that NPCs don't like a PC (for whatever reason). If PCs have to get through conversations with NPCs that can be enough to prompt some RP because the players have to converse. In one game I had a city of dwarves with a group which despised "talls"-- any non-dwarf, non-gnome. That disdain was easy to thread into dialogue and helped illustrate those elements of the setting, and my players responded to it in kind. I could also introduce rolls, as above, when it felt narratively appropriate rather than according to an arbitrary formula.

Clear this with your players first

Especially when insisting that players confront in-game biases against their characters it is important to work with those players. Not everyone is on board with dealing with this sort of thing in a game, regardless of the mechanism that you use to represent it. Discussing with your table that you think this prejudice will add to the game is a good idea in itself, and also gives you a chance to talk over any in-game implications of how you choose to represent it. That's the best advice I can give about balancing special mechanics for one player: talking about it in advance and making it clear that you're committed to making changes if it seems like the PC is overpowered or a DM favorite is infinitely better than any amount of unilateral advance planning.

Answer 3

Treat it like Wild Magic

It sounds like you want to make this more of a role-play quirk and avoid abuse; somewhat like wild magic. Since this happens on a spell attack, perhaps you could change a targeted effect into an area effect but keeping the same theme or damage type.

Examples:

• Cold/Slashing: Ice Knife transforms into a small Ice Storm or Cloud of Daggers
• Fire/Ray: Scorching Ray transforms into Fireball or Prismatic Spray
• Poison: Ray of Sickness or Poison Spray transform into Stinking Cloud
• Radiant: Guiding Bolt transforms into Moonbeam

Have multiple options for each theme/damage type to keep it from becoming predictable; which could be abused. To further prevent abuse, if the character should intentionally try to trigger the wild fireball, that's the only effect that wouldn't happen. Something else fire, or ball, related would happen.

Try to keep the power level roughly the same. If the transformed spell is too powerful, nerf either its damage output or area of effect. Use the Creating A Spell rules from the Dungeon Master's Guide (page 283-284) to keep the result balanced. If the transformed spell is not one of the character's known spells, they would not be able to use secondary effects such as being able to move a Moonbeam on later rounds.

Answer 4

Make it a level adjusted ability.

Fireball is too strong at level 1, and too weak at level 20. This would be my suggested ability track by level, with you only having the level appropriate ability.

1- burning hands 3- scorching ray 5- fireball 7 - wall of fire 9 - immolation 11 - sunbeam 13 delayed blast fireball 15 sunburst 17 meteor swarm.

That ensures the ability is always level relevant, and also adds some degree of randomness to the power, in that burning hands may be more useful than scorching ray in some situations, but you would lose access to that at level 3. Negotiation with an appropriate demon or devil or powerful tiefling might grant greater flexibility or more uses.

Don't make it random.

Random fires have a tendency to make characters confusing to use, and would discourage them from using spells, acting as a nerf. I've seen campaigns go down in flames when people have random fire spells. Instead, make it a once per day ability, with lots of demonic looking imagery around any spell use.

If you want to add drama, have them face situations where it's useful to use their ability in public. For example, have a fire vulnerable foe rampage in town, so that their ability use can save the day while exposing their demonic corruption to town people. Or, they can refuse to use their powers, trusting in their companions or their own non tiefling powers to save the day.

You can also encourage roleplay around emotional fire control, if the player enjoys it- let them make all the candles flare up when they're angry in a negotiation, without it being a formal power.

Make these abilities or similar ones accessible to others.

Who knows if the abilities are balanaced? This is an experiment. As such, the easy way to ensure balance is let other people use them. If someone expresses curiosity about these abilities, let them make a similar demonic pact to whatever the tiefling's ancestors made to gain similar abilities, or make a pact with another outsider for similar abilities.

From experience, if one player gets a special ability the others often feel resentful. If everyone can get special abilities, there's less resentment. So, ensure that demon deals are an option for all.