Instructions
Raise your Herblore skill level to level 82. You gain the Herblore ability by completing the early game quest, Druidic Ritual. From there, you can increase your skill level by creating potions and other related items.
Complete the Watchtower quest in the ogre-occupied city of Gu'Tanoth to obtain the Skavid Map. You need this map in order to enter the area that contains Nightshade. You can receive the quest from the Watchtower Wizard, who is to the northwest of Yanille.
Purchase a light source. The area you will be exploring is dark, so you need a light source in order to see your way. You can purchase candles and torches from candle sellers in various areas. However, Yanille is the best place to go, since you are in that general area when you obtain the Watchtower quest.
Enter the Skavid Caves region, which consists of six caves near Gu'Tanoth, and search for Nightshade. The herb grows in various spots in the region. Pick it up when you find it.