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?