Witcher 3 wandering in the dark
Keira will explain that there is something distorting her teleportation and demand that they leave the caverns. Destroy the rat nests with either bombs or Igni to receive 100 for each destroyed nest. Once back at the area before you took the portal, head to the right, then right again to find Keira calling for help while casting spells. The path will eventually split: the left will take you out to the main chamber where some drowners are and the right will as well, but you'll have to fight a foglet prior to the main chamber. If you did go through the portal, take out the drowners then make your way through the underwater tunnel (a chest here has a random diagram). This may be preferable to going through the portal as you will otherwise be teleported to a cavern far from the main chamber and you will have to fight drowners and foglets to get back to it to find Keira. Note: If Geralt ignores the portal he will remain in the main chamber and will begin his search for Keira from there.
Needing to catch up to them quickly, he asked Keira to open a portal to the other side, but it goes wrong and the two are separated. Follow her into the ruins where, just after you pass the second archway, Geralt will see members of the Wild Hunt on the other side of the cavern.
If you didn't head out immediately with Keira, make your way to the elven ruins, otherwise you'll arrive at them with Keira.