The Sorcerer's Empowered Spell metamagic option reads
When you roll damage for a spell, you can spend 1 sorcery point to reroll a number of the damage dice up to your Charisma modifier (minimum of one). You must use the new rolls.
While the spell chaos bolt reads:
You hurl an undulating, warbling mass of chaotic energy at one creature in range. Make a ranged spell attack against the target. On a hit, the target takes 2d8 + 1d6 damage. […] If you roll the same number on both d8s, the chaotic energy leaps from the target to a different creature of your choice within 30 feet of it. Make a new attack roll against the new target, and make a new damage roll, which could cause the chaotic energy to leap again.
If Sorcerer chooses to use Empowered Spell when they cast chaos bolt (and has a Charisma modifier of at least +2), do they need to re-roll all of the dice that they wish to re-roll at once? Or can they re-roll one d8, then, if it does not match, re-roll the other?
As an example:
The sorcerer casts chaos bolt and rolls a 7 and an 8. They use Empowered Spell, and re-roll the 7, getting a 3.
If Empowered Spell dice rolls are sequential:
The sorcerer casts chaos bolt and rolls a 7 and an 8. They use Empowered spell and re-roll the 7 getting a 3. The sorcerer uses the additional re-roll allowed by empowered spell to re-roll the 8 and gets a 3 causing the spell to jump to another target
If Empowered Spell dice rolls are rolled together:
The sorcerer already chose to just re-roll the 7 and so cannot now choose to re-roll the 8. The spell does not target another creature.
A related unofficial tweet from December 2017 by Jeremy Crawford on the subject:
When casting Chaos Bolt and using the Empowered Spell metamagic on the
first damage roll, would you be able to reroll only up to the first
two d8s, or would you be able to reroll potential secondary damage
dice too assuming your charisma modifier was high enough?You can reroll as many dice as your Charisma modifier allows.
A related question: Which yields higher average damage, an Empowered or Twinned Chaos Bolt?
Best Answer
It depends what books you are playing with
If you play without Xanathar's Guide to Everything, there are no explicit rules regarding simultaneous event resolution, so there's no particular guidance here except as regards game balance considerations (rerolling sequentially isn't a problem; empower is a relatively weak metamagic) and that whatever the DM decides goes.
If you play with Xanathar's Guide to Everything, then the following rule applies:
Rolling a die is a thing that happens. When you roll 2d6 damage, two die are being rolled at the same time. When you choose to reroll a die, that's also happening "when you roll damage", which is the same time. Thus the participant who controls the entity whose turn it is, which is usually the caster, decides what order the rerolls and original rolls which are theoretically all happening and being decided on at the same time actually happen in.