Probably a simple question with a simple answer:
When a player has Advantage - rolls twice and one is a 1, does this critical fail overrule the advantage roll?
No, a Crit Fail doesn't automatically overrule Advantage.
The Advantage and Disadvantage section of the PHB clearly states:
Use the higher of the two rolls if you have advantage... (pg.173)
Nothing in there says anything that would suggest rolling a 1 would cancel out Advantage so you would still take the higher of the two rolls.
The Rolling 1 or 20 section states:
If the d20 roll for an attack is a 1... (PHB pg.194)
So another way to look at it is that the higher of the two rolls becomes the "new" d20 roll for the attack and, so long as you haven't rolled two 1s, doesn't meet the requirements for a Critical Fail.
@mikemearls A player had advantage on an attack and rolled the first die a 15 and the second a 20, does he get a crit on the attack?
@huntinglabrats Yes - the player uses the better of the two results
Following the rules, absolutely no.
However, when it comes to disadvantage and someone rolled a natural 20, which would go to waste unless the other roll is also a natural 20. Now, according to the rules, that is just what happens - the 20 goes to waste - but some DMs will prefer counting the 20 as a crit anyways just for the sake of the players. This does mean, however, that, unless you judge a nat. 20 more of a success than a nat. 1 is a failure, you would apply the same rule to a nat. 1 on an advantage roll. Again, rule wise, (dis)advantage counts more than crits, but if the players are fine with it, the DM can choose to value crits more than the (dis)advantage or do so in specific situations, although the latter can be obnoxious to players if you don't set parameters for stakes that will allow a crit to outshine (dis)advantage.
only if you roll the 1 on the 2nd roll. if its the primary roll then it ignores all modifiers such as advantage and disadvantage.
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