Directly based off of the question, What’s the interaction between darkvision and the Eagle Aspect of the beast?, what happens if your Darkvision extends past 100 feet?
This question is focused around fact that the Eagle totem features states "You can see up to 1 mile away with no difficulty, able to discern even fine details as though looking at something no more than 100 feet away from you."
At first glance, it does appear that there is overlap between the two abilities.
Your Darkvision range is determined by whatever feature/trait/item that provides it. But they are all clear in that it is for things that are within that range.
But the question is, do they mean physically within that range or within the sight range? There really isn't anything to fully support either and it's going to depend on how a DM interprets it.
Without complicating the scenario by including the Eagle Aspect, it seems like the assumption is about distance. It's that you can see in Darkness to a distance of X.
The language around darkvision states (my emphasis):
Within a specified range, a creature with darkvision can see in darkness as if the darkness were dim light, so areas of darkness are only lightly obscured as far as that creature is concerned.
Again, it's unclear if this is about sight range or physical range. If we take it as physical range, then there is no interaction with Eagle Aspect. Things beyond 100' are still over 100' away.
But if we use it as sight range, then Eagle Aspect triggers:
You gain the eyesight of an eagle. You can see up to 1 mile away with no difficulty, able to discern even fine details as though looking at something no more than 100 feet away from you.
The things over 100' away are still over 100' away, but the Eagle Aspect has you able to see things further as though looking at something no more than 100 feet away. They aren't actually within the 100' range, but you see things that far as clearly as you would if they were actually in range.
This is the key question here: are things that you can see as if they were that close, actually that close to trigger your darkvision?
Because of the ambiguity, I do think this is going to be table dependent and to ask the DM. There are good cases for both, but it relies on determining if Darkvision is based on distance or just viewable range.
Rules as Fun seems like this would be something to approve at the table. But if the DM feels that it makes it difficult to run their campaign by providing 1 mile site at any time of day or night, then that's a reasonable call to make.
And in situations where you want to limit (if you do allow it), you can always introduce other weather effects over the concerned area.
Eagle, PHB 50:
You gain the eyesight of an eagle. You can see up to 1 mile away with no difficulty, able to discern even fine details as though looking at something no more than 100 feet away from you. Additionally, dim light doesn’t impose disadvantage on your Wisdom (Perception) checks.
Darkvision, PHB 183-184:
A monster with Darkvision can see in the dark within a specific radius. The monster can see in dim light within the radius as if it were bright light, and in Darkness as if it were dim light. The monster can’t discern color in Darkness, only shades of gray. Many creatures that live Underground have this Special sense.
The specific radius is the key factor. It stays specific to however far your Darkvision extends. There is no interaction.
If you are in Darkness there is also no interaction.
There is also no interaction that turns Darkness into bright light because Darkness only appears as dim light to you as if you looked at it. There is, however, no dim light for your eyesight of an eagle interaction to take place.
As Daniel Zastoupil made me aware, there seems to be the misconception that the Eagle aspect extends your field of vision or the range of your vision.
Rules are context-dependent: the Eagle aspect does not extend your vision. It allows you to see things in fine detail.
Visibility Outdoors, DMG 243:
When traveling outdoors, characters can see about 2 miles in any direction on a clear day, or until the point where trees, hills, or other obstructions block their view. Rain normally cuts maximum visibility down to 1 mile, and fog can cut it down to between 100 and 300 feet. On a clear day, the characters can see 40 miles if they are atop a mountain or a tall hill, or are otherwise able to look down on the area around them from a height.
No interaction means: their effects coexist without affecting each other. Thank you SilentAxe for making me aware of the term being undefined.
General clarification: Under the effects of Darkvision you can discern details at the granular levels of which you would be able to discern detail at that range under the effect of those granular levels. Darkvision does not grant you heightened awareness or any such thing.
If you have eye-injuries that prevent you from seeing fine details, then Darkvision does not grant you the ability to see fine details in its specific range.
Under the effects of the Aspect of the Beast: Eagle you would be able to discern fine details, if you formerly had eye-injuries that prevented you from seeing fine details.
The Eagle Aspect ability says:
... You can see up to 1 mile away with no difficulty, able to discern even fine details as though looking at something no more than 100 feet away from you. ...
The general effect is that you can see up to 1 mile with no difficulty. This leaves open the question of what "with no difficulty" means. This is defined further in the next sentence: you can see up to 1 mile with the same quality that you could see as if you were looking up to 100 feet away.
So, how well can you see something up to 100 feet away? Excerpting from the definition of Darkvision:
The monster can see in dim light within the radius as if it were bright light, and in Darkness as if it were dim light. The monster can’t discern color in Darkness, only shades of gray.
This is how the character can see "as though looking at something no more than 100 feet away". So therefore, within your new 1 mile radius you can see in dim light as if it were bright light, and in Darkness as if it were dim light. You may not discern different colors in Darkness.
