The Search for the Cult - NWN Main Quest (Chapter 2)
The Search for the Cult is the first story quest of Chapter 2 in the Main Campaign of Neverwinter Nights. In order to update this quest and proceed to the final part of Chapter 2 (Luskan) you need to bring 3 credible pieces of evidence to Aribeth in Port Llast. While progressing through Chapter 2 and finishing all of the side quests you'll likely find 5 or 6 pieces of evidence in total but only 3 of them are the "credible" pieces that can advance the quest.
A good rule of thumb is that the evidence you get from the many ambushes throughout this Chapter won't be considered credible. In my experience the only pieces of evidence that advance the story are the 3 journals that you find throughout this chapter. Below are the 3 pieces of evidence that are credible enough to advance the story and where they are each found.
North Road: Ganon's Journal is found inside of the Ogre Caves in the North Road map. You'll find Dergiab who is part of North Road: Dergiab's Head in the very same room as Ganon.
East Road: Relmar's Journal is obtained from the Spirit Realm in the Neverwinter Woods. To access the Spirit Realm you'll need to be on The Spirit of the Wood quest.
South Road: Charwood Cultist's Journal is found in Charwood Village by eliminating the Strange Man inside of the Charwood Inn.
For these next pieces of evidence you will typically first be approached by someone in a friendly area like the Green Griffon Inn or one of the buildings in Port Llast. They will ask you for help or threaten you and then you'll encounter them (or their friend) out in the wilderness. Soloman will typically spawn near the Green Griffon Inn. Vardoc in my experience I almost always encounter him around the Archaeologist area in East Road. Neva is the other ambusher, you'll meet Jaheel or Tarran first in a town then be ambushed by them in the South Road - Farmlands map.
Other Evidence:
Soloman: He will approach you inside of the Alliance Arms Inn in Port Llast and give you Soloman's Ring. Later on he will approach you again and give you a final warning before setting up an ambush for you at a future time. When you defeat Soloman's ambush you'll be able to loot a note off his corpse that you can bring to Aribeth.
Neva: At some point you will be approached by Jaheel or Tarran who will start the South Road: Poor Neva side quest. Once you've started this quest you'll be ambushed by Neva, Jaheel and Tarran on the South Road - Farmlands map, they will drop some evidence that counts towards this quest.
Vardoc: This event is similar to Neva in the sense that you'll have to be approached by someone before Vardoc can spawn. Benlaran, Lerner and Thurwin are the people who can/will approach you and tell you about Vardoc. I've read online that you have to talk to all 3 before Vardoc will spawn but I am not sure if that's true or always the case. One thing I can say for sure is that Vardoc spawns in a few different locations - I have seen him in at least 3 different spots myself.
Ganon's Journal inside of the Ogre Caves
Relmar who drops Relmar's Journal in the Realm of the Spirit
The Strange Man in Charwood gives you the Charwood Journal
Once you have returned 3 pieces of evidence to Aribeth she will update your quest and tell you to speak with Aarin Gend who is standing next to her. Aarin will give you some additional details about what you need to do, most importantly he'll tell you to talk to his "friend" waiting at the Luskan Gate who will let you through.
You can find Luskan by traveling straight north from Port Llast; in the North Road - Green Griffon Inn map you'll find a gate leading into Luskan in the north. If you've been to this area before you were unable to pass through the gate - now there will be a new NPC here who will let you through when you talk with him.
Important: Make sure you finish all of the quests that you want to finish before proceeding into Luskan! Once you enter Luskan you will never be able to return to this portion of Chapter 2; this is a one way trip.
The gate leading into Luskan
Inside Luskan you'll find Aarin Gend inside of the Temple of Tyr, you'll have to talk with him here to update this quest. For quick travel to and from the Temple of Tyr use your Recall Stone, the destination it brings you during this part of Chapter 2 will be the Temple of Tyr in Luskan and not the one in Port Llast.
Aarin Gend will inform you that Aribeth has gone missing and that your main goal in Luskan is to get inside of the Host Tower. To get inside the Host Tower you'll first need to obtain a High Captain's Seal which will allow Aarin to forge the documents needed. You can obtain the seal we need during The High Captains Story Quest.
For the time being this story quest is basically put on hold. You won't make any progress with it until after you've entered the Host Tower. To progress the main story at this point in time you'll want to focus entirely on The High Captains Story Quest.
Aarin Gend inside of the Temple of Tyr
Once you have obtained a High Captain's Seal from either of the High Captains all you need to do is return to Aarin Gend inside the Temple of Tyr and give it to him so he can forge the documents we need. You can access the Host Tower by giving the forged documents to the Host Tower Bridge Guard who is found in the northwestern portion of Luskan.
There will be a guard who stops you on your way into the Host Tower, it doesn't really matter what you say to him in my experience. Inside you'll have to visit Aribeth's Room which will have a hostile Shield Guardian golem inside of it, loot the Tower Entry Key from inside of the Armoire and use it to unlock the door leading into the southern large room.
In the southern room you'll find a pillar named Host Tower Portal, when you click on it a single entry will come up for the 2nd Floor. Clicking on 2nd Floor entry will make a portal appear nearby which you'll want to enter to get started with this dungeon. Every floor will have a pillar named Host Tower Portal and a circle where the portal will appear once you click on a specific floor. A screenshot of what I am talking about is shown below.
How the Host Tower dungeon works is pretty straight forward. Throughout the dungeon you will find various __ Floor Portal Stones; they start with a 3rd Floor Portal Stone and will go all the way up to a 9th Floor Portal Stone with one exception, the Pinnacle Portal Stone. As the name suggests, the Pinnacle Portal Stone will take you to the pinnacle of this dungeon which is the final floor and where we need to go.
Each floor of the Host Tower is a small dungeon typically with an event that is unique to that floor. I won't explain each floor since basically every single one of them is completely optional. You should explore every floor because there's a lot of enemies for EXP throughout the tower and probably a dozen or more magical items.
All you need to know for advancing the story is that most of the floors will have Portal Stones you can find that will allow you to access the other floors. There will be a few duplicate Portal Stones too so incase you miss one in one area you can always find another in a different location.
Like I said before, to reach the final floor of this dungeon you'll have to get the Pinnacle Portal Stone. This item will allow you to teleport up to Level 9 which is the second to last level (and connected to the pinnacle). Below are the 2 different locations that you can find the Pinnacle Portal Stones. Keep in mind that you only need to obtain a single one of these stones and not both.
Pinnacle Portal Stone Location #1: Host Tower 6th Floor; in the northern portion of this floor you'll find Valindra Shadowmantle who is standing outside of a small bedroom which contains the Portal Stone. Inside the bedroom you'll find a large trap along with a Familiar enemy that you'll have to dispatch.
Pinnacle Portal Stone Location #2: Host Tower 7th Floor; this one drops from Warden Erjack who is found in the northern portion of the 7th floor. He's a big Minotaur and pretty hard to miss, along with the Pinnacle Portal Stone he drops various other magical items.
Host Tower 9th Floor
Once you make it to Level 9 you'll find a skeleton named Arklem and 4 rooms each with a Brazier. What you need to do for this part is smash each Brazier then speak with Arklem to unlock the door leading up to the pinnacle. Each time you smash a Brazier an enemy will be summoned at the circle next to Arklem. At first the enemies are pretty easy but it will progressively get harder as we go.
Your final fight will be against a Death Slaad which will be a pretty powerful enemy and may be hard to beat depending on your level and build. I'd recommend resting up and returning to the Temple of Tyr if you need to restock any supplies as the next fight can be a challenging one too.
For the final fight of Chapter 2 you'll be facing a large group of enemies ontop of Host Tower (pictured below). When the fight is over be sure to search the small room that's down the nearby staircase. There will be a Stone Golem in this room along with a chest you can check for magical items. Sometimes it has goodies, other times it has nothing but vendor trash.
Now all that's left of Chapter 2 and this quest is to return to Aarin Gend in the Temple of Tyr and speak with him. Go through his dialogue and you'll be all done with this quest and moving onto Chapter 3!