Contract: Missing Brother
You will learn about this quest by reading the note after interacting with one of the notice boards in Velen. Go to the Inn by the Crossroads (M2,4) and talk to Brun who is the quest giver of this contract. Before you take the job, you can bargain for your payment for completing it.
Now you can go to the mine North-East from the inn. Activate your witcher senses and enter the cave. Follow the red markings. Because it is dark in the cave, it is wise to help yourself with the Cat potion. Examine the blood marks while exploring the cave and reach the place where Geralt must dive. Swim under water to be able to continue the walk.
Soon after you walk out of the water, you will start noticing some young ones. Eliminate them and look around for the eggs from which they hatch. Examine any egg and then start destroying them with Igni sign (burn them). Four eggs can be found close to you, eight other can be found in the side corridors (it's not required for completing the quest).
Before you enter the main cavern, save the game, make sure that Geralt has at least 30th experience level and make some preparations (replenishing health, placing the insectoid oil on the weapon, etc.). You will have to face the giant spider Harrisi. Once the battle starts, focus on attacking the large spider, but from time to time take care of the young arachas that can get in your way (they will appear if you haven't destroyed the eggs). The monster can poison you or drag close to it. Additionally, you must watch out for powerful attacks performed with its limbs - it is best to jump towards the monster, deal few hits and jump away. After a victorious battle, examine the spider's corpse and take the trophy (5% chance of finding additional herbals). You can go back to Brun and tell him the bad news about his brother's fate. The man will not want to pay Geralt the sum you have agreed on. You can take half the payment from him or use the upgraded version of Axii sign (special dialogue option - it requires fully developed delusion ability) and force him to give you the full payment.