Creepy, but good fun.
The Witcher 3: Blood and Wine - What Lies Unseen
Travel with Regis or meet him at the entrance to the Unseen Elder's lair. There's a Place of Power nearby you can scramble up to before you go in, if you fancy it; if it's your first visit, you'll get an ability point.
After opening the door, head through and jump into the pool of water. Dive down to swim through the tunnel, hanging left until you can surface; the path to the right leads to an air pocket but is a dead end.
When you climb out of the water look around for a loot chest at the edge of the pool. Keep following the path around until you reach a lit tunnel on your left; the opening to your right leads back to the entrance, and Regis will likely hop through. (You can climb up here yourself but you don’t get sweet vampire gauntlets.)
Climb the ledges to head down the lit path. Ignore the dead end on the left and move onwards. A cutscene begins as you approach the room with multiple lights. There’s a timed choice: you can threaten the elder or say you’re not there to fight. It doesn’t seem to matter either way.
When you regain control, follow the prompt to collect some gear (use Witcher Senses to spot it easily) and then heal yourself. You’ll pass out. When you wake up, collect any remaining gear and head down the tunnel, looting the container you pass on the right.
Drop down into the large cavern and head to the waterfalls. Examine the one flowing upwards, loot the container, collect and read the notes, and examine the footprints.
From the waterfall, hug the wall and travel in a clockwise direction, past the area where you dropped down to enter this room, and travel down the tunnel you come to (the mage’s footprints lead this way).
If you would like the rest of the Han Gaidth set we started collecting in this area, jump up the ledge leading to the first tunnel on your left and explore to the end to meet three protofleders. They hit extremely hard, but they’re otherwise just like fleders. They're guarding two pieces of the set in a loot chest and a couple of minor loot containers.
Return to the main tunnel with the mage’s footprints and continue on to fight two more protofleders. There are some minor loot containers in this room. Vault the ledges and continue following the footprints.
Loot the container on your left as you pass through the doorway, then collect and read the mage’s notes on your right. Examine the megascope and loot two nearby loot containers.
Continue to follow the footprints and things get weird. Move forward and collect the loot – the chest on the right contains tow more pieces of the set. Continue to follow the stream, vaulting the ledge and collecting the loot container on your right. Cross the gap and battle the protofleder, then examine the mage’s body. Take the nearby sword, read the notes, then search for the offering near the low wall at the edge of the platform. There’s also another minor loot container here.
Cross back over to the previous ledge and then leap across to the higher central platform. Follow the path around and use Aard to destroy the column that glows red against your Witcher Senses. Jump down into the stream and walk into the waterfall at the other end.
Back in the first area, use the fallen column to climb to a new ledge. Follow the tunnel along and eventually you’ll enter a cutscene conversation.
Most of the choices in this conversation are literal dead ends. If you waste time exploring optional and incorrect answers, you’ll get a game over screen. The only options that will get you to your goal are as follows:
- First prompt: Here About Detlaff.
- Second prompt: Detlaff spells trouble for you.
- Third prompt: either choice
In the next scene you can explore some optional dialogue with Regis to cement your friendship and learn more about higher vampires, if you like. In any case, you're off to the final missions now.