oh you to get the light source you need to do either everything up to the purification lake in the lizard tribe story line or talk to the witch at anytime I think and she'll ask for the glowshroom in front of the cave you can't enter. A nd the cave is unlocked by using the glowing amulet you get from on an accessory slot and trying to go in. But the lizard priestess also will turn them into an amulet which can be used in an accessory slot for light to enter. But for the lake you need to find all the pieces of the statue of the lizard, which one is given by a quest from the witch during the day, another is found in the bandits camp and the last is the head from when you first encountered rhot. (Wait coming back to this message you might just need glowsrooms and the priestess or witch to unlock one....)