Wandering In The Dark - Velen
When completing the Hunting a Witch quest, if you did not head out immediately following Keira then you'll have to meet her at the entrance of the Elven Ruins in the southern portion of Velen on your own. It's quite a long run down south but the exploration on the way there is good for you.
Once you arrive at Keira's location speak with her and follow her inside of the cave. Very shortly after entering you'll encounter the Wild Hunt and Keira will make a portal for you to go into (pictured above). You have an option of going into the portal or not, either way you have to look for Keira.
I'd actually recommend you take the portal as you'll find an Alchemy Recipe in the chest that you pass under water. You'll find the room with Keira a little bit to the northeast of where we entered - there will be two Monster Nests marked on your map at her location.
Tip: To get rid of the poison gas you can use Aard (to blow it away) or Igni (to blow everything up). Be warned that the poison gas is flammable, don't stand it in when you blow it up.
Tip x2: Spend some time exploring the ruins, there is some treasures for you to find here that will not be covered in this quest summary.
After saving Keira and exploring the ruins, follow the quest marker on your map/compass. You'll come across a projection which leaves a clue for Ciri that Geralt deciphers into follow the Swallow. In the next part of the cave you will find markings on the wall, follow the one of a swallow to continue with the quest. I recommend you search the other routes too for treasures.
In the room with the second projection your quest will update and tell you to find a marker of Ciri's Horse, Kelpie. The mark that you're looking for is below you just above the water, as pictured in the screen shot below. You will have to jump down to reach it.
Swim through the underwater passage after inspecting the Kelpie symbol and regroup with Keira to continue the quest. It won't be long until you stumble upon a portal by two Swallow markings on the wall that will teleport you to our next location for this quest.
You'll have to fight your first boss of this dungeon shortly after the portal, a Golem (pictured below)! Elementa Oil as well as Dimeritium Bombs work well against the Golem, if you have any on you. Otherwise, just remember the 5 Ds of dodgeball and you'll do just fine against this boss. This is also great advice for the game as a whole - keep mobile during fights and you'll almost always win.
The next boss of this dungeon is optional, a Gargoyle. Go the opposite direction of the quest marker to fight this enemy and claim the treasures in its room and on its corpse. The Gargoyle is weak to the same things as the Golem and the fight is very similar, it's slow and you're going to want to dodge all of its attacks. Strike when Keira stuns it if you're on a very high difficulty.
Our next challenge will be to protect Keira as she closes the portals that the Wild Hunt opened (pictured below). You will have to stay close to her, within her bubble, and follow her around the room. Each portal will spawn pairs of hounds that you'll want to take out before they deal too much damage to Keira. If you want to preemptively take out the hounds you can toss a Dimeritium Bomb at the portal too - this will prevent the hounds from even spawning.
After closing all three of the portals Keira will collapse into your arms and you will be given the option of two responses...
1. Just don't faint on me (Raises romance level)
2. If you can't go on (Does not raise romance level)
Shortly after the room you protect Keira in with opening portals you'll come across the cracked wall that's pictured above. You're able to use Aard and blow this wall away which will give you access to some treasures. I'd recommend grabbing this now while we're here.
In the upcoming room you'll be fighting Nitral, a member of the Wild Hunt and the final boss of this quest. This fight is similar to the fight against the Golem and Gargoyle, all of these attackers are very slow with their attacks and Keira will occasionally help you throughout with stuns. As for symbols, use Quen during this fight - the boss is resistant to just about everything else.
Periodically throughout the fight Nitral will create a bubble around himself and open up a portal that summons Hounds of the Wild Hunt. He'll also port to the middle of the area. Attacking Nitral during this time or getting close to him will cause Geralt to take a decent chunk of damage so don't do it. The only way to drop Nitral's shield is by clearing out the Hounds of the Wild Hunt that he summons.
Once Nitral is defeated and before proceeding with the quest you will want investigate everything in the room and collect all of the loot you can. Head over to Keria when you're ready to proceed with the quest. At this time you'll receive the next story quest, Ladies of the Wood, however there is more for us to do with this quest first.
Three things will be glowing red near Keira, some herbs, a potion, a book and the wall. Inspect them all and conclude with the wall to discover it's an illusion, Keira will give you The Eye of Nehaleni which allows you to dispell this illusion as well as any other illusions you come across. Handy!
Tip: On the opposite side of the room as the elf's laboratory you'll find a cracked wall that you can blow away with Aard. Behind that cracked wall is a large rock which you can dispell using The Eye of Nehaleni. This will lead you to a chest guarded by a Foglet with a powerful sword.
Follow Keira to the exit to end this quest. A new quest will begin immediately following this one called Magic Lamp. This quest is optional but it can only be done right now, if you agree to help then leave you'll fail the quest. I highly recommend you do this quest as the Magic Lamp is required for some future quests.
Continue to Magic Lamp Quest Walkthrough **DO THIS NOW**