During Act I, you will be on your way to the Underdark after progressing through the early parts of the main story and the Zhentarim Basement is one of the ways to get there.
The Zhentarim Basement, which is also referred to as the Zhentarim Hideout is where a group of mercenaries can be found and interacted with.
Being one of the ways to the Underdark, the Zhentarim Basement gives you the chance to obtain certain equipment that may be helpful once you get there.
How To Get To The Zhentarim Basement?
To get to the Zhentarim Basement, you will first need to head to Waukeen’s Rest which is located north of the Blighted Village and other areas.
Once you cross over to Waukeen’s Rest, you will be able to find a town that has been attacked and may find a storage room near a barn that you must enter.
The entrance (Shabby Door) is blocked off by crates that you can move or destroy and once you head inside, a cutscene will begin.
A man named Salazon will be waiting for you to enter, thinking that you are the one who attacked their village.
You will need to pass an Ability Check to prevent Salazon from casting Fire Bolt as this will hit several explosive objects.
Talking Salazon into backing down will result in him giving you the key to a hidden entrance (Engraved Key) that opens a passage behind a Wardrobe.
Interact with the Wardrobe to use the key and you may take the stairs that have now been revealed, which will take you to the Zhentarim Hideout.
Entering the Zhentarim Hideout will start a cutscene once you take a few steps in, which requires you to speak with Zarys to deactivate any traps along the way.
What is in The Zhentarim Basement?
1) Find The Missing Shipment Quest
If you happen to encounter Rugan and Olly before reaching the Zhentarim Basement, you will later find that they have been captured if you stole the Iron Flask.
This results in them being captured by the Zhentarim, who appear to have killed Olly and are currently torturing Rugan.
2) Brem’s Secret Shop
One of the NPCs in the Zhentarim Basement may be interacted with to obtain special items from his “special stock”, providing you with new items to purchase.
These items appear as you gain the trust of Zarys, which can be done by saving Rugan and Olly, allowing them to deliver their shipment.
3) Passage To The Underdark
Within the Zhentarim Basement is a passage to the Underdark, which can be accessed by going past an illusionary wall.
Once you get past the illusionary wall, there is a lift that can be unlocked and used to reach the Underdark.
Verdict
Getting to the Zhentarim Basement may not be the easiest path but there is a lot you can find along the way to the Underdark from there.
Being on the good side of Zarys will give you access to decent gear that you may purchase from Brem that may be useful for some of your characters.
Whether you decide to side with the Zhentarim in their hideout, you will be able to access the elevator that leads to the Underdark.