Okay, I have a Wood-Elf character that is going to become a Monk that follows the Way of Shadow. A Way of Shadow Monk gets the ability to cast the spell Darkvision (among others) using Ki. I want to know how the spell interacts with races that already have dark-vision. Does it add to the races natural dark-vision? or Does the spell simply overlap?
[RPG] How does the spell Darkvision interact with races who have darkvision naturally
dnd-5eracial-traitsspellsvision-and-light
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Well, if you don't mind making a pact with some powerful entity, you could get 2 levels of warlock. This will give you access to a few spells, some of which perhaps being interesting (both stats and flavor-wise) for your character. But most of all, this will give you access to 2 invocations.
One of these invocations should be Devil's Sight, which grants you the ability to see through any form of darkness, magical or otherwise, for up to 120 feet. This is more than enough to cover the 15 feet of the darkness spell. Cast it on your clothes, and bring terror to your enemies.
While this is a good strategy (generally favored by blade pact warlocks), it has some limitations. While under the effect of the darkness spell, your allies cannot see you. As such, any spell requiring line of sight that they would like to cast on you simply cannot work. Healing you might be problematic, as it requires your allies to be able to touch you. They might know that you are smack in the middle of that sphere of pure black darkness but, once they get inside, it's suddenly not so easy to find you anymore.
These limitations also affect your enemies, however. Any attack from the outside of the sphere should have disadvantage as your enemies, even if they can approximate your position, can't exactly see you to aim properly. Enemies inside the darkness should also suffer from disadvantage when attacking you, unless they possess some form of blindsight or a similar ability. When you attack them, you should have advantage on the roll since you are functionally invisible for them.
The biggest cost of this strategy is the fact that it requires multi-classing. While the lvl 19 and 20 abilities for monk would not really be missed (realistically, few campaigns will reach these levels), it will still set you back 2 levels in obtaining you other core monk powers. Note that doing this would get you 2 invocations, however, and that some of them can be quite interesting for a ninja-esque character:
Armor of Shadows (cast mage armor at will, which might be better than using your wisdom for AC if its under 16),
Eldritch Sight (cast detect magic at will),
- Eyes of the Runekeeper (allows you to read anything, even languages you don't know... useful for spying!),
- Gaze of Two Minds (share the eyes of a willing target, more spying!),
- Mask of Many Faces (disguise self at will!), etc...
TL;DR
Sacrifice 2 monk levels and multiclass as a warlock. Gain some minor but potentially useful spellcasting, as well as the ability to see through magical darkness and one more ability of your choice!
Yes, it is possible but...
The number of ways you can go about it is limited to somehow seeing through magical darkness like by getting Truesight.
Or by picking up 2 levels of Warlock for the Devil's Sight Eldritch Invocation, in order for you to see through magical darkness up to 120 feet.
Or by any means like the 6th-level Divination Spell True Seeing, or a Gem of Seeing (page 172 of the DMG) also does the trick, or you can homebrew a magic item that serves this purpose.
What is important is that you can see the area of magical darkness/dim light created, all others might still see it as magical darkness, that doesn't matter.
However, the bottom line is that both of these features were not meant to provide a mini-portal for the monk to Shadow Step through because Shadow Step, by its wording:
Shadow Step
At 6th level, you gain the ability to step from one shadow into another. When you are in an area of dim light or darkness, as a Bonus Action you can teleport up to 60 feet to an unoccupied space you can see that is also in dim light or darkness...
Emphasis on shadow here, we can see that the intent was for the Monk to step through shadows, not magical darkness. I know this sounds silly but if we look at the next emphasis, "you can see", it affirms this intent by limiting the area a Monk can step through to an area he can see. Hence, these two were not meant to be used in-tandem due to its incompatible properties. However, there are many uses for Darkness and Shadow Step alone in themselves. Darkness can be used to Blind enemies, causing them to have Disadvantage on Attack Rolls. One clever trick is also to use Darkness to cover a room's light source like a single torch, Darkness negates that source and engulfs the whole room in natural darkness, giving any creature with Darkvision advantage. Shadow Step can enable the Monk to avoid stepping into a brightly-lit portion of a room (and thereby avoid being seen) by teleporting to a dimly-lit portion.
Don't misunderstand me, stepping through magical darkness can be done but you'll have to work for it
Best Answer
You would gain no additional effect.
There is nothing in the spell's description that says you gain anything but 60 feet of darkvision. It is widely accepted that spells do only what they say they do.
Darkvision PHB p230:
Compare that to the Goggles of Night DMG p172
If your DM wishes to give additional benefits because "its magic" that would be a house rule.
Personally, I don't really do that sort of thing because I look at it in a way that some spells were created by creatures that don't have such abilities and the base spell would not have considered a creature with that ability.
However, it is plausible that a variant would have been developed by a society consisting of those with darkvision that would extend it, but that would not be a common spell in my thinking and would have to be found while adventuring. That spell might not grant any benefit to a creature without natural darkvision.