A quick reminder on the visibility rules in D&D 5e:
- Lightly Obscured: dim light, patchy fog, moderate foliage; creatures have disadvantage on Wisdom (Perception) checks that rely on sight.
- Heavily Obscured: darkness, opaque fog, dense foliage; creatures suffer from the blinded condition.
- Blinded: creature automatically fails any ability check that requires sight. Attack rolls against the creature have advantage, and the creature’s attack rolls have disadvantage.
- Darkvision: 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 the creature is concerned.