Contract: Missing Brother - Velen
Location: Inn at the Crossroads
Quest Giver: Bruno/Notice Boards throughout Velen
Other Requirements: N/A
This quest begins by speaking with Bruno who can be found inside the Inn at the Crossroads in Velen. After speaking with him you'll need to travel northeast to the mine's entrance, it's basically across the water and a tad bit south of Border Post. Here your quest will update and you'll need to head inside.
Before following the foot prints and proceeding through the underwater tunnel you'll want to search the cave thoroughly. Inside you'll find a few Crafting Diagrams amongst other goodies - if you're having trouble locating anything use your Witcher Senses or a Cat's Eye Potion. Located in one of the center rooms with a baby Arachnas you'll find a ladder you can climb up that leads to a lever (pictured below).
This lever will drop a draw bridge that leads back to the cave's entrance. Basically, this dungeon is one big circle with a middle room that connects back to the cave entrance. Keep this in mind while you're exploring it.
To proceed with the dungeon you'll want to take the underwater passage that's nearby the middle room with the ladder which will take you to the room shown in the screen shot below. Here you'll want to inspect the huge blood spot on the ground to update your quest and deduce that the enemy we're after is a Venomous Arachnas.
You will also receive a scent trail after inspecting the blood which will lead into the next underwater passage; this route will lead you to a dead end with nothing there. Instead where you're going to want to go is west in the room that's pictured below, this will lead you to the boss of the area.
Additionally any eggs that you see, like the ones in the picture below, you'll want to use Igni on. The reason it's important for you to destroy all of the eggs is because during the fight with Harrisi she'll spawn them all and for every egg left alive you'll have to deal with an additional little spider.
Harrisi has three main attacks; the first of which is a web she spits out that latches onto Geralt and pulls him to her. The second thing she likes to do is lob a bunch of toxic projectiles around the room and her last special move is to fart out a large poison cloud. Use Quen every time you need it and try to stay behind her/at her side. Keep an eye on her butt and when it's filled she's about to use the poison cloud.
Harrisi Weaknesses: Golden Oriole (Potion), Insectoid Oil
After Harrisi is dead be sure to examine the room thoroughly - there is a rare sword for you to find here in addition to the loot on her body. When you return to town to turn in the quest you'll be given three options, each one will give you a different reward.
1. Tell him you risked your life (earn 200 EXP/84 Crowns)
2. Use Axii (Earn 40 EXP for Axii and 200 EXP for quest/210 Crowns)
3. Say you won't beg (300 EXP - no Crowns)