The Gambling downtime rules in XGtE state that it takes the character a workweek (5 days) and a stake of between 10gp and 1000gp to partake in the gambling downtime activity.
The resolution of this activity (according to the XGtE rules) is three skill challenges are attempted by the player, against three randomly generated DCs (DC = 5+2d10 for each check).
Wizards of the Diviner School have the feature Portent which gives them:
When you finish a long rest, roll two d20s and record the numbers rolled. You can replace any attack roll, saving throw, or ability check made by you or a creature that you can see with one of these foretelling rolls. You must choose to do so before the roll, and you can replace a roll in this way only once per turn.
Each foretelling roll can be used only once. When you finish a long rest, you lose any unused foretelling rolls.
As the gambling downtime takes place over 5 days, the question arises over when exactly do these checks occur? Is it all on a single day, spead out over the 5 days?
This question is important, as it directly affects how much of the Portent ability is available for the player to use.
My instinct is to spread the checks over the 5 days, and use this method:
- Player declares in advance how much they want to gamble (their stake)
- The DM rolls a set of DCs for the three checks for each day (using the 5+2d10 method)
- The diviner would then roll their portent for each day, and choose one or more of the checks (if any) to use their portent on. They can also choose just to chance it and do any number of "regular" rolls on a given day.
- Passing or failing the check would be resolved by comparing the result of their roll(s) against the generated DCs for the day for the relevant checks.
- If a check is failed or passed on a given day it is passed or failed for the whole session
- If by the end of the 5 days they still have outstanding checks left to complete, these would be resolved by straight ability check rolls against the "average" DC for the session (calculated for each skill using 5 + Average(DC/day – 5)). These checks would not be eligible targets for Portent as they are multi day checks.
Would this ruling be balanced (and/or exploitable)?
To be clear here, the houserule is the daily "sets" of DCs being generated and the "average" set at thee end, instead of the set XGtE makes out for the 5 day activity as a whole.
Best Answer
That ruling heavily favors the diviner
When you're rolling DCs privately and randomly, this has no real effect on the player's choices. Because I have no information on what you've rolled behind the DM screen, it does not affect my actions. As such, we'll ignore the randomness of the DC and just focus on the wizard's side.
Assuming level 2 divination wizard, they will have 2 foretelling rolls per day, totalling 10 rolls over the 5 days. Because you allow them to forgo making any checks on a day they don't like their Portent chances, this is only slightly worse than "roll 10 take highest 3" (it's slightly worse because if you get 15s one day and forgo the check in the hopes of something higher, you can't go back to 15). This kind of selection is very strong.
This is not to say it's unbalanced, because you as the DM set what happens. If you're in a setting where magic is normal, people might know exactly what diviners can do, and even if you aren't, consistently "lucky" hands will tip off anyone that pays attention.
You might represent this as a higher than 10 percent chance to run into a gambling complication, or the overriding of a chance, and you just say "it happens."
However,
When you make the checks isn't clear
So your ruling is as valid as any, because it's left up to DM purview.