Dueling Portents

Question

I have a player with a Divination School Wizard. He has used his Portent ability upon the Big Bad several times to great effect. My Big Bad is a smart person however, and is seeking to hire a rival diviner to account for this monkey wrench in his plans.

I'm not sure of the proper way to adjudicate dueling Portents. Does the first Diviner state their intent, followed by the second diviner, with the second diviner trumping the first? (my first instinct). Should their be a skill challenge of some sort? Higher level diviner? The first diviner cannot expend his second Portent die to trump the trumper, since the ability specifies only one usage per turn.

The difficulty here is that it's a very meta ability as written. There's no action economy other than once per turn, no range, no requirement to see or even be aware of the target or its intent. As far as I can tell, a diviner can even use the ability when surprised.

I don't think there's a RAW answer to this, but something supported by or induced from RAW would be be appreciated.

Answer 1

Comes down to when the Wizards chose to use their Portent

The main concern here is going to be Metagaming. The Diviner chooses to use their portent before the roll as shown here (emphasis mine):

You can replace any attack roll, saving throw, or ability check made by you or a creature that you can see with one of these foretelling rolls. You must choose to do so before the roll, and you can replace a roll in this way only once per turn.

There is no visual or auditory clue that this is happening in game. Just that the Divination Wizard has predetermined the outcome.

NPC Foil

Given that you are creating the NPC as a foil to the PC and that PC is choosing when to exercise their Portent, you will need to have done the same with your NPC. Waiting until after the PC does it is metagaming and would be frustrating for the player and remove their agency.

However, if you choose to use portent prior to the roll and before the PC has decided to use theirs, then I see no concerns and you are using it in the way prescribed.

Simultaneous Effects can be determined by Creature Turn

Per Jeremy Crawford, it is possible for two portents to be used on a single roll:

Different wizards can use Portent (a School of Divination feature) on the same roll. See Xanathar's Guide to Everything (p. 77) for guidance on how to adjudicate simultaneous effects like that.

Xanathar's suggests:

Most effects in the game happen in succession, following an order set by the rules or the DM. In rare cases, effects can happen at the same time, especially at the start or end of a creature’s turn. If two or more things happen at the same time on a character or monster’s turn, the person at the game table — whether player or DM — who controls that creature decides the order in which those things happen. For example, if two effects occur at the end of a player character’s turn, the player decides which of the two effects happens first.

If you have chosen to use the NPC portent simultaneously with the PC Portent, the order of Portents will be determined by whomever's turn it currently is.

If it is an NPC turn(Portent on the Bad Guy), the DM chooses the order , but if it's a player's turn(Portent on an ally), then the player will choose.

Please see my warning above about metagaming with regard to player agency, though.