In Mouldwood Depths (before defeating Mora) and in the cave at Glades, you need a light source in order to navigate through the darkness. The game expects you to either use the fireflies or flash but there is other abilities which can be used as a light source.
Bow
The easiest way is to use Bow. As long as you are holding an arrow, your Bow will light your surounding which prevents the darkness from killing you and resets the death timer.
Grenade
You can also use Grenade as a light source, which is much harder since it doesn't protect you while you are holding Grenade. You need to throw it near you to refresh the darkness cooldown with the explosion and launch another one almost immediatly since Grenade's light won't protect you for long.