What does the Underdark Scout feature of the UA revised ranger's Deep Stalker Conclave do?

Question

In Unearthed Arcana: The Ranger, Revised, the Deep Stalker Conclave subclass has a feature called "Underdark Scout". The description of this feature states:

You are also adept at evading creatures that rely on darkvision. Such creatures gain no benefit when attempting to detect you in dark and dim conditions. Additionally, when the DM determines if you can hide from a creature, that creature gains no benefit from its darkvision.

I don't quite understand this. When a creature has darkvision, it can see in the dark like it's dim light (PHB, p. 183-185):

a creature with darkvision can see in darkness as if the darkness were dim light.

And dim light creates a "lightly obscured area" (PHB, p. 183; emphasis mine):

a lightly obscured area, such as dim light, patchy fog, or moderate foliage, creatures have disadvantage on Wisdom (Perception) checks that rely on sight.

Then the only benefit that darkvision gives is the ability to see, right?

So how I do interpret this? Should I rule that they simply don't have darkvision against the ranger?

That would make the ranger like a "heavily obscured area", so any creature trying to locate the ranger while in darkness effectively has the "blinded" condition against him (just as if the ranger had invisibility). Is that correct?

Answer 1

Not quite as invisibility since it has its own caveats, emphasis mine.

You are also adept at evading creatures that rely on darkvision. Such creatures gain no benefit when attempting to detect you in dark and dim conditions. Additionally, when the DM determines if you can hide from a creature, that creature gains no benefit from its darkvision. UA: The ranger, revised P. 8

The main difference is that, invisibility does not end when you are detected by other means that are not magical or with special senses but, in the ranger case, the condition/ability ends as soon as the ranger is detected.

The invisibility condition in PHB 191.
An invisible creature is impossible to see without the aid of magic or a special sense. For the purpose of hiding, the creature is heavily obscured. The creature’s location can be detected by any noise it makes or any tracks it leaves.

Attack rolls against the creature have disadvantage, and the creature’s attack rolls have advantage.

Now, about detection; attacking or casting a spell make you lose the benefit as stated in PHB. 195. Also, there is a big difference between dark and dim conditions (PHB. 183). The main difference is that with normal sight you can see the ranger in dim conditions but you are blinded in dark conditions. Therefore, the ranger in dim condition is not treated as if it has invisibility/heavily obscured, the ranger is treated as if in dim condition.

(PHB 195) If you are hidden—both unseen and unheard—when you make an attack, you give away your location when the attack hits or misses.

(PHB 183) A given area might be lightly or heavily obscured. In a lightly obscured area, such as dim light, patchy fog, or moderate foliage, creatures have disadvantage on Wisdom (Perception) checks that rely on sight.
A heavily obscured area—such as darkness, opaque fog, or dense foliage—blocks vision entirely. A creature in a heavily obscured area effectively suffers from the blinded condition.

Thus, against the ranger when trying to detect him in dark or dim light condition, a Dark Elf and a Human are not treated differently, as if they have the same visual capacities.

As an example, by courtesy of Keithcurtis; Robbie the Ranger and Frank the Fighter are trying to sneak past Danny the Drow and Harry the Human in a dimly lit cavern. Harry has disadvantage on the opposing perception on both the sneakers, while Danny only has disadvantage against Robbie the Ranger.