Description -
This cave is built to hold captives until they are sent to Menzoberranzan to be sold as slaves.
The gate to the slave pen is kept locked. A character using Thieves' tools can pick the lock with a successful DC 20 Dexterity check. A character using makeshift tools can attempt the same check but has disadvantage. A lock-picking attempt might draw the attention of the guards, requiring a Dexterity (Stealth) check contested by the guards' passive Wisdom (Perception) score to carry it off without notice. Each of the guards on duty in the other areas of the outpost has a key to the gate hanging from a belt ring. Breaking the gate's lock and forcing it open requires a successful DC 20 Strength check.
Magical Wards -
The Drow have placed powerful wards on the slave pen to inhibit spellcasters and shield the area against scrying attempts.
Spells cast within the slave pen have no effect, and any slot or magic item charge expended to cast such a spell is consumed. The wards don't suppress or negate spell effects that originate outside the slave pen. For example, a creature under the effect of an Invisibility spell remains invisible when it enters the slave pen.
Creatures inside the slave pen can't be targeted by any Divination magic or perceived through magical scrying sensors.