Case, the SRDSorry, but the difference is just huge.
Rolling a die is completely different from rolling an attack roll.
Halfling's Lucky
Lucky. When you roll a 1 on an attack roll, ability check, or saving throw, you can reroll the die. You must use the new result, even if it is a 1.
Sorcerer's Unerring Spell
If you make an attack roll for a spell and miss, you can spend 2 sorcery points to reroll the attack roll. You must use the result of the second roll.
Case 1)
Halfling's luck normal attack roll is 2d20 (If you roll a 1). Rerolling a die
Sorcerer's Unerring Spelln ormal attack roll is 2d20 (If you miss). Rerolling an normal attack roll
Case 2)
Halfling's Luck advantage attack roll is 3d20. Rerolling a dice
Sorcerer's Unerring Spell advantage attack roll is 4d20. Rerolling an ADVANTAGE attack roll (Pick the best roll).
Case 3)
Halfling's Luck disadvantage attack roll is 3d20. Rerolling a dice
Sorcerer's Unerring disadvantage attack roll is 4d40. Rerollar a disadvantage attack roll. (Pick the worst roll)
Completely different from Lucky feat or Halfling's luck, Unerring Spell rerolls an entire attack roll, not just a die.
It's clear as water. It's 6d20.
nothing about it apply to reroll an attack roll. Did you undertand it? Halfling's lucky trait is just a die. Unerring Spell doesn't reroll the d20, It rerolls the attack roll.Case, the SRD
When you have advantage or disadvantage and something in the game, such as the halfling’s Lucky trait, lets you reroll the d20, you can reroll only one of the dice. You choose which one.
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nothing about it apply to reroll an attack roll. Did you undertand it? Halfling's lucky trait is just a die.
Nothing about an attack roll isn't a d20. Even if you have advantage, the "attack roll" by rule is the single d20. Per the SRD.
What makes you happy at your table is fine. But your build counts on an exploit that ignores rules that are specifically addressed as counter to your interpretation.
You did a specific build and using specific material (Eberron) to try to get around Sleep and Poison damage (Partially), but you completely forgot that the sorcerer is still a sorcerer with metamagic. Fireball, Hypnotic Pattern, Dragon's Breath and other spells simply kill that character quickly.
From now on, your character has completely lost its source of damage without weapons (he even lost his ability using Int modifier in the roll attack) and incapacited for 1 minute. I don't need lists on how to defeat this character because he doesn't stand a chance and that's not the point here either.
Also, correcting here, while the artificer uses a greatersword, he cannot cast Absorb Elements or Shield. (Somatic Component).
CR 8 is supposed to be an average challenge to level 8 party. Not a single character level 5. Right?
Nope, @Flamestrike is correct. Specific beats general, and their is a specific rule that says that reroll abilities only ever reroll one of the dice.It's an attack. Unerring Spell Repeat all attack modifiers, not a d20.
It is, but it's specifically a medium encounter for a party of four 8th level characters. It becomes an easy encounter with 5 PCs, and trivial for 6.Wrong.
A single CR 8 creature is not even a 'medium' difficulty encounter for a group of 8th level PCs.