When exactly is the outcome of a roll determined?

Question

So, while thinking on the recent Lucky/Portent question, something has come to my mind:

The Lucky Feat states

You can choose to spend one of your luck points after you roll the die, but before the outcome is determined.

When exactly is the outcome determined?

I can see how it is not determined in scenarios where there are still unkown variables in place, such as the Attack Bonus or AC from a monster the party is fighting against, or the DC of a challenge they have to overcome. But I don't see where I draw the line in some other scenarios.

Some examples of these scenarios:

• If you make an attack roll and roll a 20, the outcome is instantly determined - you hit. It doesn't matter the enemy's AC or the bonus you are going to apply, the outcome will be the same (as in 'it is already determined').
• If you are making a Death Saving throw, the instant you roll the outcome is determined. You either succeed or fail, without any bonus to apply. The DC is also known and fixed (10), so there is no unknown variable.

Even if the outcome is determined in the same moment you roll the die (i.e. "After you roll, before the outcome is determined" doesn't exist) it seems obviously intended that you can still use the Lucky feat. It wouldn't make much sense that you can't use Lucky to change an enemy's attack roll just because it was a Nat 20 (confirmed by Crawford) and Lucky can be used in Death Saving Throws.

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As a bonus:

• For the DM, the outcome is determined the instant he sees the roll for (almost) everything. He even knows exactly the outcome (not only 'it hits', but 'how, when and where it hits'). Can a DM ever use an NPC with the Lucky feat?

Answer 1

When the DM describes the outcome

An outcome is determined only when the DM describes the outcome, not before. This is built into the fundamental game loop of D&D: DM describes the world, players take actions, DM determines outcomes of these actions (maybe using dice rolls).

It may be obvious that if you roll a 20 for an attack, you crit, or that when you roll a 1 on a skill check, you have almost certainly failed. But the outcome being obvious/inevitable does not mean it has yet happened.

As a bonus, here's one thing which might make one of your "instantly determined" examples not such a foregone conclusion:

Illusory Step. Beginning at 10th level, you can create an illusory duplicate of yourself as an instant, almost instinctual reaction to danger. When a creature makes an attack roll against you, you can use your reaction to interpose the illusory duplicate between the attacker and yourself. The attack automatically misses you, then the illusion dissipates.

Once you use this feature, you can't use it again until you finish a short or long rest.

So this feature could mean a 20 on an attack roll misses.

And more generally there might always be something (RAW or not) that can change the outcome of what looks like a "determined" d20 roll (Portent being the most obvious thing).