As the title suggests, me and some friends had a disussion on this. First, here is the RAW from PHB on Portent (PHB 116):
Starting at 2nd leel when you choose this school, glimpses of the future begin to press in on your awareness. When you finish a long rest, roll two d20s and record the numbers rolled. 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. Each foretelling roll can be used only once. When you finish a long rest, you lose any unused foretelling rolls.
And the spell description of nondetection (PHB 263):
For the duration, you hide a target that you touch from divination magic. The target can be a willing creature or a place or an object no larger than 10 feet in any dimension. The target can't be targeted by any divination magic or perceived through magical scrying sensors.
Now let's say, to make this a little easier to decide, instead of casting nondetection which has a duration the target is simply wearing an Amulet of Proof Against Detection and Location which has exactly the same description(DMG 150):
While wearing this amulet, you are hidden from divination magic. You can't be targeted by such magic or perceived through magical scrying sensors.
Now we can eliminate the "When did the wizard actually see this person in a vision and did he have nondetection up at that time" topic.
The main thing my friend insisted on was that "Portent isn't tagged as divination magic.", "It's not a spell." or my favourite "It's not magic, it's precognition."
Lastly, he gave this example in Portent's favor:
A thief wearing the amulet tries to steal a cup. Now the diviner can use portent to affect the thief's roll because he can see the cup doesn't go anywhere or the cup won't be there in couple seconds.
I believe if there is a nondetectable target in the area the whole thing is supposed to be fuzzy and the amulet should cover anything that target interacts with for some time.
We had a lot of different opinions on how things were supposed to be but you get the idea.
What's the final verdict?
Before continuing i want to say this didn't happen in a game or anything, just a random thought while reading the abilities, in a game i'd probably let ignore the divination magic part in some cases for player favor as you suggested. But as a general rule i want to keep asking the things that just don't click.
"Speaking purely Rules As Written, Portent is a class ability for Wizards who choose the Divination School, but it isn't classified under Divination Magic."
Then what is it? it's definitely not precognition since you are supposed to see the events during your rest. Psychics are way out of question here as well.
Whether it's a class feature or ability, there is no doubt there is magic involved in Portent and it's quite visibly placed under the School of Divination. The way i see it, it is Divination Magic in RAW. Not a spell obviously, that's why the nondetection description says magic and not spell.
"However, I would take it under advisement that the Portent isn't just targeting the creature under Nondetection, but it is also targeting the interaction."
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 still have to be able to target the creature before you even think about the roll.
"Let's say that the wizard has cost Hold Person on someone under Nondetection and plans to use their Portent on the saving throw. Is it that the Wizard has seen the outcome of their Spell or that they've seen the outcome of the effect on the target? It could be read either way."
I think this example is a bit flawed since there is no roll for casting the spell, only the outcome of it on the target.
Sometimes the mechanic and the game logic has to be kept separate.
I believe in this case both work on the favor of nondetection. Let me explain:
Mechanic:
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Portent is Divination Magic(back to that point are we)
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Nondetection says you can't be targeted by Divination Magic
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Portent needs a target.
Logic:
- With portent you get glimpses of the future during your long rest. If there is someone protected you simply don't see — not just them but also — their effects on the environment. It's safe to say the hidden person's interactions are also hidden.
Since players can't really see the future they might learn the target is protected when they try to use Portent, saying they saw what happened. I'm thinking the DM would say somewhere along the lines of "This person wasn't in your visions", "That dagger wasn't stuck in your friend but now it is, enemy must've found a way around your divination magic" and you'd keep the Portent charge.
Best Answer
Rules as written, it is left to DM or the table for interpretation
Rules as written what you're supposed to do is look at Nondetection and Portent and decide for your table how it works. You're supposed to look at what the intent of Nondetection and the intent of Portent is supposed to be, which means you'll have to read and decide for yourself based on how you understand the rules and what works best for the fun and enjoyment of your table. That is 5e's rules philosophy.
I don't buy the "It doesn't say it is or it isn't, therefore it isn't." You can just as easily say, "It doesn't say it is or it isn't, therefore it is." Lack of evidence doesn't prove anything. The game rules do not claim to be comprehensive, and this is the natural consequence of that. This is the entire point the "rulings not rules" idiom is making. There is no burden of proof on Portent or Nondetection or the PHB as a whole to provide an answer. While that line of reasoning was implicit to 3e and 4e -- both editions that sought comprehensive and complete rules sets -- 5e actively does not do that. 5e D&D is often intentionally vague so as to leave rules open for interpretation and the system intentionally doesn't use keywords or raw mechanics. It does this so that players and DMs have the explicit freedom to interpret the rules for themselves and do what makes sense for that interaction. The designers know they can't possibly foresee every interaction in the rules, so they no longer try.
There is no clear answer by design. Yes, this means that asking, "What is RAW?" on RPG SE for 5e D&D is often a pointless exercise because the answer you should often get is, "RAW it depends on your table." This is why there are so many conflicting answers on RPG SE for 5e questions and why Sage Advice contradicts itself so often.
The rules are less important than the game. What Mearls and Crawford want is for players and DMs to stop asking WotC how to play and just make a decisions and play for themselves. If you make a mistake, admit it and make a correction. It's no less destructive than doing nothing until WotC makes a decision and then maybe changes their mind later.
You're expected to look at whole picture that the rules are giving you and to make a judgement call on what feels the most consistent and correct for your table. Is it reasonable for Nondetection to block Portent? Sure, it almost certainly is divination magic given that it's an ability of the Diviner subclass. How about a Ranger's Primeval Awareness? Well, that works like a spell, even consuming a spell slot, and it would have to be divination magic given the distance, so sure. How about a Paladin's Divine Sense? Hm... possibly, it's pretty close to Detect Good and Evil, but it's really described as the Paladin's senses. A Warlock's Devil's Sight? Hrm, hard to say. It's got elements that only True Seeing can accomplish, and Nondetection probably blocks True Seeing, but it's basically an improved Darkvision spell and that's not even Divination. So maybe partially? A Barbarian's Feral Instinct? Eh, that doesn't seem right, it's not magical. A Rogue's Blindsense? Yeah, probably not unless Blindsense is supposed to be magical, but I don't get that impression.