Underdark

Find the Zhentarim Hideout: First off, you need to locate the hideout itself. The entrance is often found near a specific guard animal. The guide mentions it's hidden behind an illusion, so keep an eye out for anything suspicious.

Get the Mechanism Key: Inside the hideout, you'll need to find an NPC named Karad. He's usually patrolling the lower levels towards the back. Your goal is to steal the 'Mechanism Key' from him. This is where your stealth and pickpocketing skills come in. You'll need a successful pickpocket roll with a DC (Difficulty Class) of 10. If you mess this up, things could get dicey. Alternatively, if you're feeling particularly ruthless, you could always take him out, but be mindful of the consequences.

Locate the Elevator: Once you have the key, make your way towards the guard animal named Ishtor. You'll want to position your party in stealth mode, hiding behind Ishtor so you're out of sight from both the animal and Karad (if he's still around). The guards in the main hideout shouldn't be able to see you here either.

Bypass the Illusion and Use the Elevator: Get close to the wall where the elevator is supposed to be. You might notice it's an illusion. Even if the guards don't spot it, you can move through it. With the 'Mechanism Key' in hand, interact with 'The Elevator' and use the key. This will activate it and transport you down to the Underdark.

Arrival in the Underdark: You'll emerge in the Underdark near the Selunite Outpost. You can reach this area by solving the Moon Puzzle in the Defiled Temple. Once you've used the Zhentarim Outpost Elevator, you can use it again anytime you want to travel between the hideout and the Underdark.