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The sixth level Circle of the Stars feature, Cosmic Omen, allows the druid to, for the remainder of a day, use their reaction to either help their allies or hinder their foes:

Whenever you finish a long rest, you can consult your Star Map for omens. When you do so, roll a die. Until you finish your next long rest, you gain access to a special reaction based on whether you rolled an even or an odd number on the die:

Weal (even). Whenever a creature you can see within 30 feet of you is about to make an attack roll, a saving throw, or an ability check, you can use your reaction to roll a d6 and add the number rolled to the total.

Woe (odd). Whenever a creature you can see within 30 feet of you is about to make an attack roll, a saving throw, or an ability check, you can use your reaction to roll a d6 and subtract the number rolled from the total.

Cosmic Omen, Tasha's Cauldron of Everything, pg. 38

Note, however, that in the description of this feature, the player is simply instructed to roll "a die", without qualifying what kind of die it has to be. This poses no issue for the "standard issue" dice used in 5e D&D, since the d4, d6, d8, d10, d12, and d20 all have an even number of sides, and therefore an [assumed, with fair dice] equal probability of rolling an even or odd number.

However, there are dice that have odd numbers of sides. In this situation, a player could pick up a three-sided die, for example, and they'd have a 2/3rds chance of rolling an odd number, biasing their feature towards hindering enemies.

Is there any rule in 5th Edition D&D that forbids a player from choosing to do this?

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    \$\begingroup\$ By extension you could have a custom die with only odd numbers. Such dice exist, but are probably also excluded from expected dice owned: propdog.co.uk/Odd-Throwing-dice \$\endgroup\$ – AncientSwordRage Mar 7 at 20:21
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    \$\begingroup\$ @AncientSwordRage My assumption here is that any n-sided die that doesn't have sides [1,n] wouldn't be allowed, since otherwise the "tens" die, which is standard in most sets of 7 dice, would be permissible, and while the game doesn't "assume" you'd have that, it's an extremely common die to have. If it were allowed, then you'd only be able to roll even values. \$\endgroup\$ – Xirema Mar 7 at 20:53
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    \$\begingroup\$ @AncientSwordRage en.wikipedia.org/wiki/Intransitive_dice \$\endgroup\$ – nick012000 Mar 8 at 6:18
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    \$\begingroup\$ Do you feel that in the next edition, the rules writers need to specify "don't cheat" as a rule? I am having trouble understanding where this question is coming from. \$\endgroup\$ – KorvinStarmast Mar 8 at 13:54
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    \$\begingroup\$ The game munchkin has a card which says something like "Roll a die. You can use any dice you available", with the best outcome obtained by rolling a one. So of course I got myself a blank die and labeled it with ones on all sides. But that game specifically encourages such behavior as a satire on people that behave like this in more normal games. So take from this what you may. \$\endgroup\$ – mlk Mar 8 at 20:08
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The fact that it says to roll a die, but doesn't say which one, indicates that it's not supposed to matter.

In general, any mechanic that tells you to roll a die tells you what kind. In this case you're not told which die to roll, or which ones you can choose from, or even that you are supposed to choose one. This strongly indicates that there's no gameplay choice being made, and the mechanic expects all dice to be equivalent, which all the standard dice are.

If you're allowed to just invent arbitrary kinds of dice, then why pick a d3? Why not roll a "d1" and get an odd result every time?

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  • \$\begingroup\$ Because a d3 is a legitimate die (as in they actually exist), not a made up one. If someone owned a d3, and the player pulled it out of the bag as the random die to roll, that's legitimate. Yes, you could make a transparent sphere with the numeral 1 visible inside, or a custom d6 with a 1 on all sides, but that's not really a legitimate die and there is no reason have it other than a joke or to cheat. \$\endgroup\$ – Xavon_Wrentaile Mar 8 at 1:38
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    \$\begingroup\$ @Xavon_Wrentaile There's also no reason to use a d3 for this roll other than as a joke or to cheat. Whether there might be some legitimate use for it in a different game is beside the point. \$\endgroup\$ – Mark Wells Mar 8 at 2:22
  • \$\begingroup\$ @Xavon_Wrentaile The game Dice Forge is an example where there are legitimate reasons for having a die where all the faces are equal. Can't really have one with all faces having the same value due to lack of faces. Bit if more of the same faces were added to extend from 4 to 6 players then a player could end up with a die where all the faces are the same and it would make sense for the mechanics of that game to have such a die, it would be a legitimate die. The player could save the effort of rolling it in that case, of course, and just take the ensured outcome when a roll is needed. \$\endgroup\$ – Anonymous Coward Mar 9 at 13:32
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The rules assume you only have a standard polyhedral set.

It seems abundantly clear to me that the intent is for the die to have an even numbers of sides. The feature’s outcomes are based on odd or even, and a standard polyhedral set consists of an even numbers of sides on every piece.

And most importantly, the rules assume you only have a polyhedral set. From the introduction to the Player’s Handbook:

The game uses polyhedral dice with different numbers of sides. You can find dice like these in game stores and in many bookstores.

In these rules, the different dice are referred to by the letter d followed by the number of sides: d4, d6, d8, d10, d12, and d20. For instance, a d6 is a six-sided die (the typical cube that many games use).

Then, the introduction goes on to explain how to simulate dice that are not a part of the standard polyhedral set:

The same d notation appears in the expressions “1d3” and “1d2.” To simulate the roll of 1d3, roll a d6 and divide the number rolled by 2 (round up).

So this rule is assuming again that you don’t actually have a d3.

Finally, the rules describe how to simulate a d2, and this proves that the rules are assuming that you have no odd sided dice at the table:

To simulate the roll of 1d2, roll any die and assign a 1 or 2 to the roll depending on whether it was odd or even.

Here again we see the ambiguous “any die”, much like the Stars Druid’s ambiguous “roll a die”. If the rules really meant for this to include dice with an odd number of sides, then this suggestion makes no sense at all, since we are trying to simulate a d2.

The intent of the rules are abundantly clear: roll a die with an even number of sides.

You're probably not going to break anything by allowing this, as the features are mostly symmetrical anyway.

Mechanically, nothing is going to break if the DM allows the player to manipulate rolls this way. Both features are usually going to be equally useful. In fact, the inclusion of both attack roll and saving throw makes Weal and Woe essentially symmetrical. Both can be used in defending and attacking.

Weal can be used to make an attack more likely to hit, and woe can be used to make a saving throw effect more likely to go through. Woe can be used to make a hit less likely to hit and Weal can be used to make a saving throw effect less likely to go through. The parity of attacks and saving throws here makes these two features mostly indistinguishable.

But you might be trying to cheese powerful spells.

The only foreseeable issue here is that Woe can be used to strategize around high level spell effects that call for saving throws. The return on a failed save for high leveled spells is significantly greater than adding some bonus to hit on one attack. If your party is working with a caster that has some big bang saving throw spells, there would be no reason not to do this if it were permitted.

As a DM, I wouldn't allow this; in my mind, it isn't faithful to the theme of the feature.

The feature describes the thematic element is is after:

Whenever you finish a long rest, you can consult your Star Map for omens.

The idea here seems to be that the stars are telling us something, and adding player agency to this seems inconsistent thematically.

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    \$\begingroup\$ @gatherer818 It has instructions for simulating a d3; you still can't actually roll one. \$\endgroup\$ – Mark Wells Mar 7 at 20:52
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    \$\begingroup\$ @gatherer818 The rules explicitly explain how to simulate a d3 using a d6 because the rules are assuming you don’t have a d3. \$\endgroup\$ – Thomas Markov Mar 7 at 22:13
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    \$\begingroup\$ @Xirema No, because it’s just a matter of common sense. If you as a DM or the DM you play with wants to permit this, that’s obviously fine. But the rules are written in such a way that it’s simply not possible that this feature is intended to permit odd sided dice. \$\endgroup\$ – Thomas Markov Mar 8 at 0:26
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    \$\begingroup\$ That's actually kind of my point though: it's not clear the "rules as written" actually forbid this, at least from what you've quoted. I only see an assumption that the player wouldn't be able to do this. \$\endgroup\$ – Xirema Mar 8 at 2:52
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    \$\begingroup\$ @Xirema And my point is that it is so abundantly clear that you can’t do this that there is no point in mentioning that you can’t do this. \$\endgroup\$ – Thomas Markov Mar 8 at 7:21
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Not At My Table

As the question stands at the time of this answer, you're really asking two questions:

  1. Can I do X? (No)
  2. Is there any specific rule which forbids X? (Also, no.)

These are being asked as though they are obviously equivalent up to a negation-- that is, as though if one is answered 'Yes' the other must obviously be 'No' and vice-versa. But that isn't so.

The reason why there is no specific rule against X (in this case, non-standard dice) is because the set of standard dice is exhaustively defined already, and it is redundant to explicitly rule against all others.

Aside from that, just because a rule isn't specified doesn't mean it is not commonly understood and enforceable.

  • Does the PHB specify that the dice have to be fair? Not as far as I can see.

  • Can you shave or load dice at my table? Absolutely not.

  • Can you use a d20 with little '20' stickers on the other 19 faces? Absolutely not.

  • Can you use one of those specialty d10s labelled 10, 20, 30... intended to make d% rolls easier? Sure, because I give permission at my table to use them for that purpose. But you can't use them here and interpret that as an all-even die.

The intent of those rules is clearly to simulate a coin flip. Why didn't they say coin flip? Why didn't they say 'd2'? I don't know, and I don't care, but it's abundantly clear what they meant.

At my table, this gets met with a cold over-the-spectacles stare, and nothing more.

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    \$\begingroup\$ This is the best answer here. Rule 0 exists for the DM to clear up ambiguities (among other reasons). Don't try to explain what the designers might or might not have intended. Tell them they need to roll a fair, even sided die. Grab one of my d30s, fine. Find my one of d5s or d7s, no. \$\endgroup\$ – Xavon_Wrentaile Mar 8 at 1:42
  • \$\begingroup\$ So my intention was to ask the latter question. \$\endgroup\$ – Xirema Mar 8 at 2:53
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    \$\begingroup\$ @Xirema I figured, that's why I downvoted it. \$\endgroup\$ – Novak Mar 8 at 3:09
  • \$\begingroup\$ And this is why the game of Nomic has rule 101, "obey the current rules", and pretty much no other game needs to say that out loud. (BTW, now I want a d7 to pick a random day of the week.) \$\endgroup\$ – Codes with Hammer Mar 10 at 14:26
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Personally, I’d rule ‘roll any die’ means any regularly numbered (1,2,...) die with even number of sides to make the chances same whatever even sided regularly numbered die is rolled.

But... if you can convince your DM you are permitted to use ‘any’ die, you’ll want to obtain a doubling cube from backgammon. All numbers on it’s faces are even.

BTW, there are d1s available from shapeways: möbius strips. I also recall seeing small klein bottles that could also be used as a d1s; they’re sold as a pair of earrings, but are not hard to detach

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    \$\begingroup\$ Do you let your players do this? Has your DM allowed this at your table? (I laughed out loud at the Backgamon doubling cube reference - been a few years since I played) \$\endgroup\$ – KorvinStarmast Mar 8 at 13:55
  • \$\begingroup\$ I chucked at the idea of using a möbius strip. \$\endgroup\$ – Codes with Hammer Mar 10 at 14:25

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