Geralt has had enough of Dettalff's emotional outburst and his violence against Beauclair, which is why he's risking his life to seek out the Unseen Elder with Regis. What Lies Unseen in The Witcher 3: Blood and Wine is the penultimate main quest for the story, and Geralt might not know it, but the fates of many have forever changed with his decision to visit this ancient vampire. Below, you'll find a full walkthrough so you know what choices to make and how to get the Hen Gaidth Armor Set.
If you choose to see the Unseen Elder, then you will be locked into "What Lies Unseen", and there will be no way to go back for a different outcome in the Blood and Wine story without reloading a save.
The Unseen Elder's Lair
Take your boat south, across the waters. You will eventually reach land, where you can take the stairs to find Regis at a cave wall. You can unlock this wall with the Magic Stone acquired from Orianna, and inside, you'll find a path on the left or right. Take the right path, as both lead to a Fleder you need to kill.
Hen Gaidth Gauntlets & Steel Sword Location
Before you proceed to the main path, run and jump to the ledge in the east corner. Follow the cave to the right, and you will find a chest that contains the Hen Gaidth Gauntlets and the Hen Gaidth Steel Sword.
You can now return to the Fleder room and climb the ledge where the light source is coming from. This will start a cutscene with the Unseen Elder, where you have two dialogue options, but both result in Geralt being bitten and thrown into a deeper part of the cave.
Find a Healing Potion & Collect Your Gear
When you wake up, you will need to grab a healing potion before your health runs out. Use Witcher Senses and look in the eastern area of the cave for Superior Swallow, which you can use by pressing R1, then double-tapping Up on the D-Pad.
You will then wake up a few hours later, but make sure you pick up your swords and crossbow before exiting the room!
Find a Way to Appease the Unseen Elder
Follow through the cave, and by the waterfall you will find a backpack containing the Mage's notes, which you need to read. By the campfire, you will also be able to examine footprints with Witcher Senses, which you can then follow to a part of the cave that splits to the left or right.
Hen Gaidth Mask and Trousers Location
Take the ledge to the left, and you will have to fight three Protofleders. This cave is a dead end, but in the south area, you will find a chest containing the Hen Gaidth Mask and the Hen Gaidth Trousers. With those acquired, return to the lower area of the cave to continue following the mage's tracks.
Hen Gaidth Armor and Boots Location
You will eventually find a megascope and more Mage's Notes, which you need to read. Now, follow the waterfall's stream, and you will have a cutscene of you walking up the ceiling. Right in front of you is the final chest containing the Hen Gaidth Armor and the Hen Gaidth Boots.
Wearing the full Hen Gaidth set will restore 6% Vitality for every kill. There is also no Silver Sword in the set.
Continue on the path, and you will find the corpse of the mage, along with a Crumpled Letter and a unique silver sword, the Cantata. You will then be able to pick up the Offering for the Unseen Elder. With this key item acquired, use Aard on the stone pillar and jump into the waterfall, where you can then return to the Unseen Elder.
Speak With the Unseen Elder (Correct Answers)
The Unseen Elder is not a patient vampire, and if you respond incorrectly, he will instantly kill you and force you to reload a previous save. To spare yourself the trouble, this is what you should say to the Unseen Elder:
Here about Dettlaff. Do not select any optional dialogue, as the Unseen Elder will kill you.
Dettlaff spells trouble for you. Do not ask for help or say innocents are dying, as the Unseen Elder will kill you.
To convince Dettlaff to spare the city. This choice doesn't actually matter, as the outcome is the same as saying Dettlaff has to die.
Once Geralt and Regis leave the Unseen Elder's lair, they will have a moment to talk. There's no consequence to worry about when speaking with Regis, but selecting the optional dialogue is nice for the Witcher and the vampire to connect further after what they just experienced.
Completing "What Lies Unseen" will immediately put you into the next mission, "Tesham Mutna", and you will be placed directly into the boss fight with Dettalff, with no alternate ending to focus on, unlike "Beyond Hill and Dale…"
Source: Game Rant