How does the Flames of Phlegethos racial feat interact with the sorcerer's Metamagic option Empowered Spell?

Question

If I have both the Flames of Phlegethos feat and the sorcerer's Metamagic option Empowered Spell, can I re-roll the same die twice?

The description of the second benefit of the Flames of Phlegethos tiefling racial feat says (XGtE, p. 74):

When you roll fire damage for a spell you cast, you can reroll any roll of 1 on the fire damage dice, but you must use the new roll, even if it is another 1.

The description of the sorcerer's Metamagic option Empowered Spell reads (PHB, p. 102):

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.

You can use Empowered Spell even if you have already used a different Metamagic option during the casting of the spell.

Can you re-roll the same die with both features, or only once? That is, if you re-roll a die with one of these abilities, does the other ability let you re-roll it?

The "You must use the new roll" makes it sound like you cannot, but in general abilities can can override how other abilities work. Is the "keeping the new roll" restriction limited to re-rolls from that ability, or does it apply to all re-rolls from any source?

There is another interpretation of "You must use the new roll." There is a feat called Savage Attacker which reads:

Once per turn when you roll damage for a melee weapon attack, you can reroll the weapon’s damage dice and use either total.

If you compare this to the language in the previous two abilities, you see that the "re-roll and must use the new roll" could be viewed as the alternate to "re-roll and use either roll", and therefore imposes no restriction on other features re-rolling the die.

Answer 1

I believe that in both cases, the "you must use the new roll" should be interpreted only within the context of the effect itself. That is to say, the intent is that the Flames of Phlegethos effect can only reroll a given die once; you don't get to keep rerolling 1s until you come up with something that isn't a 1. But then once all that rerolling is done, you can decide to activate Empowered Spell and start a new round of rerolling under new rules, including additional rerolls of any dice you may have already rerolled with Flames.

However, I would say you need to fully resolve one of the effects before you start in with the other one. If you rolled a 1, then used Flames to reroll it and came up with a 2, then decided to activate Empowered Spell, and upon rerolling your 2, got a 1 again, you couldn't then go back and claim another Flames reroll. You've already resolved that effect.

Answer 2

You may use both abilities for the damage roll, but not together on any dice affected by the other ability.

Both abilities explicitly state, "you must use the new roll."

RAW (Rules As Written), this is pretty straightforward. Whatever you roll on the re-roll, you must use it. This means if you roll a 1 on a fire damage die and use Flames of Phlegethos to re-roll it, you may not use any other re-roll abilities on this specific die because "you must use the new roll" of the fire damage die. This would also apply to the use of Empowered Spell being used first, then wishing to utilize Flames of Phlegethos afterward.

You may use both during the same spell damage roll, but not together on any dice. As in, you may re-roll any fire damage rolls of 1 with Flames of Phlegethos, and any other damage die with Empowered Spell, but you may not use Empowered Spell on any dice you re-rolled using Flames of Phlegethos, and you may not use Flames of Phlegethos to re-roll any dice you used Empowered Spell on.