what happens if two characters use portent?

[FabulousFizban]

for instance, if the DM creates an enemy with class levels, and both the enemy and a PC try to use portent.

[TyGuy]

for instance, if the DM creates an enemy with class levels, and both the enemy and a PC try to use portent.

I guess it's shenanigans like counterspelling counterspell

[No brains]

It causes:
Record scratch. Freeze frame. You're probably wondering how I got into this situation.

I just don't think this is something a DM should do. Portenting a player's save to a 1 would feel unfair.

I think if this were to be done at all, one use of portent can undo another. Say you have a newbie player who doesn't get when to use portent. a DM can have an enemy use the ability to show them how it is done.

[Particle_Man]

Well pcs win initiative ties with npcs so I guess they also win “portent offs”.

[Teaguethebean]

I would say it is whoever said it first as portent must be used before the dice is rolled meaning when someone uses portent that invalidates the chance to use portent. Now the real question is could you portent someone while they are determining their portent dice.

[Greywander]

You have to use Portent before the roll is made. I'd rule it as using Portent counts as making the roll, and thus a second character can't also use Portent on the same roll, because the "roll" has already been made.

If two characters try to use Portent on the same roll, I'd say whoever declares it first wins. I'd be lenient with the players if I was the DM, though, and if one of them was debating using Portent, I would wait for them to decide instead of trying to jump in and use it first.

[Ironheart]

I think the big deciding factor of how portent could be used on the DM's side of the screen is how visible the roll is. If the DM rolls the portent rolls openly, then one could expect them to be treated like legendary resistances-- something the players have to burn through in order to have an easier time to deal with the character that has portent rolls. Balance-wise, I think having portents on two sides should work fine. The only real issue I see is the "You can't do that thing you just rolled a 20 on because I replaced it with a 5." which can lead to DM vs. Player mentality. I think I would allow a monster to have it if it just affected attack rolls and ability checks. The big key here is that the players know the rolls that they could have to deal with, and so getting advantage is important-- you want to force the baddy to use his portents on things that you can make a lot of rolls on, like a fighter with Extra Attack.

One issue I see cropping up is the DM keeping the rolls hidden, which is technically within the DM's rights - it's not like the party was there when the monster finished his long rest and got the Portent dice, and it can add a bigger sense of risk. Do you go for risky plays like using GWM for bigger damage bonus if the creature can mess up your roll? I think it gets a bit too meta for my taste, but I do enjoy the mental image of two wizards trying to alter the future and being imposed on one another. These are the reality shifting shenanigans that wizards want.

Also, bardic inspiration might be worth looking at as well as a countermeasure-- adding a d8 to a d12 to a tampered roll might nullify the portent dice altogether.

RAW: The creature that decides to use their portent dice first on a creature they can see runs the risk of the opposing creature replacing that roll, the only restriction being that each creature can only do so once a turn. These portent rolls need to be declared before the target rolls the appropriate check, however.

It could be ruled that once a roll has been replaced, that satisfies the condition of a roll being made and so perhaps this interaction can't exist.

Edit: Ninja'd by Greywander :P

[Witty Username]

Second portent replaces the first is how I would do it, but I would let someone use guided strike to overcome a shield spell so I may just be weird.