A 14th level Monk has a Diamond Soul.
Whenever you make a saving throw and fail, you can spend 1 ki point to reroll it and take the second result.
A 9th level Fighter has Indomitable.
You can reroll a saving throw that
you fail. If you do so, you must use the new roll.
The Lucky feat states that:
You have 3 luck points. Whenever you make an attack roll, an ability check, or a saving throw, you can spend one luck point to roll an additional d20. You can choose to spend one of your luck points after you roll the die, but before the outcome is determined.
So, if a Fighter or a Monk re-rolls their Saving Throw, does that mean they cannot then use Lucky? Or is the wording just meant to say you don't pick one of the 2 values, but instead just the new one?
As an example:
Fighter rolls a WIS Throw, rolls 1 and DM informs him he failed the save. Fighter uses Indomitable and rerolls, getting a 10. Before the DM tells him whether he succeeded or not, can the Fighter decide to roll an additional Luck die, or must he take that 2nd Indomitable roll?
Best Answer
You can spend Luck on the first roll or the re-roll, or both.
Read the rules carefully. You "spend one of your luck points after you roll the die, but before the outcome is determined". So you use Luck before you have failed the saving throw.
Diamond Soul lets you spend a ki point to reroll whenever you make a saving throw and fail. That's after the outcome is determined. The same goes for Indomitable.
So if you use a Luck point, you get to roll one additional die to determine the outcome of the roll. If you then don't succeed on the saving throw, you've "made a saving throw and failed". So then you can use Diamond Soul or Indomitable to re-roll. If you want, you can use Luck on the re-roll too, before the outcome of that is determined.
Note that if you roll a Lucky die, you can choose to ignore it. It's not a "re-roll" so you don't have to keep it.
Example: Fighter rolls a Wisdom Saving Throw, rolls 1, and decides not to use Luck because they have Indomitable. The DM informs them the save failed. The Fighter uses Indomitable and re-rolls, getting a 10. Before the DM tells them whether they succeeded or not, the Fighter can decide to roll an additional Luck die.