My ally is up against a Berserker who used Reckless Attack for Advantage, and hit with a nat20. I use Runic Shield to make them re-roll. Do they re-roll with Advantage, or do they just roll a single new d20 and use that result?
It should be a single d20 and use the new result. It specifies to reroll the d20 rather than to reroll the attack and requires that you use that roll, making the other d20 from advantage irrelevant.
My ally is up against a Berserker who used Reckless Attack for Advantage, and hit with a nat20. I use Runic Shield to make them re-roll. Do they re-roll with Advantage, or do they just roll a single new d20 and use that result?
It should be a single d20 and use the new result. It specifies to reroll the d20 rather than to reroll the attack and requires that you use that roll, making the other d20 from advantage irrelevant.
It technically only rerolls the 1 die that they chose from advantage.