UNDERGROUND PASS QUEST WALKTHROUGH
Talk to King Lathas upstairs in East Ardougne Castle. King Lathas will tell you about Iban who claims to be the son of Zamorak and will ask you to meet his main tracker Koftic who can be found at a cave located in the far west of Ardougne. It is best to have food and armour before attempting this quest.
Unlike the other quests all you need to do is walk through the gates into west Ardougne, just enter and go west and you will run into Koftic. You can only enter West Ardougne if you have completed the Biohazard quest. Koftic explains to you that you will need to go through the Underground Pass filled with Zamorak's spirits. He wants you to meet him at the bridge inside the cave.
When you enter the cave you will run into three branching roads, do not use the middle one, go to the rock slide on the right and climb over it there will be three rock slides on the road you will need to climb over. After you pass that part you will see Koftic again, speak to him.
Koftic will tell you that it looks like others have been in the cave before you and that he has found some clothing among some burned arrows, he also tells you that he found a book that is a diary for an adventurer named Randas.
Koftic will give you a wet piece of clothing, use an arrow with it, use the clothed arrow on the fire and you will get a lit arrow. Now you need to shoot the lit arrow at the guide rope on the bridge across the river. To do this go into the room to the north with your bow and arrow equipped and take your shot and you should automatically walk over the bridge. Do not pull the lever unless you want to walk back to the other side. Go into the room north of the bridge and take a plank, you will need this later on.
Proceed through the cave, you will run into an area with two paths, one is a swamp that you won't be able to cross, and the other a big hole in the ground. Go to the hole and use your rope on the rock pole in the ceiling, and you will hopefully swing across. If you fail you fall into an area full of rocks it will hurt a bit, so have food on this part. All you have to do is follow the path and climb over some rock slides, you should find the end of the trail eventually.
Follow the path and you should find Koftic, talk with him and then try to cross the grid. The combination is different from person to person, so just keep trying until you get to the other side. If you fail here, you will fall down and take heavy damage, have some food ready here as well. Once you're across, pull the lever, and you will walk through the big port.
Keep following the path, you should reach some zombies. Right after them you need to go through a narrow path, watch out, there are traps in the walls. If you have high enough thieving, you should be able to disarm the traps on the walls, if not, you'll just have to run through them, and pray to Zamorak/Saradomin/Guthix that you don't get hit too much. Now you will be in a room with an altar, a well (don't climb down it yet), and four paths (other than the one you came from). You need the four Orbs of Light.
Path 1 (northern): Here you need to use your plank that you hopefully got earlier, on the flat stones on the ground and you should cross the stone. 
Path 2 (north-western): Fight your way through the monsters, and pick up the orb. 
Path 3 (middle-west): Same way as path 1, just use your plank on the flat stones. 
Path 4 (south-western): Just go all the way in, but before you pick up the Orb, search the stone under it to disarm the trap. 
Once you have all four orbs, go back to the zombies (don't forget the traps in the wall), and you should see a big stone furnace to the north and use your Orbs of Light on it. Now you can finally climb down the well in the other room. If you have room, search the crates to the west for some food. Keep following the path and you will end up at a prison like area.
Pick the lock in the south-eastern cage and use your spade on the mud in there, follow the new path to the ledge, cross it (you will only lose few hit points if you fail), and then you'll come to a giant maze. However, if you are level 50+ thieving, you can just go through the cage to the south, which is a lot easier. If you are under level 50 thieving, you'll have to take the long way through the maze.
You should now be at a place with a few rats and some giant bats. Go south to an Obstacle pipe and climb through it. You should now be in a room with a unicorn in a cage. Search the cage for a piece of the loose railing, and go up the path that leads to the back of the cage. Use your piece of railing on the boulder there, and it will fall down and kill the unicorn.
Go back down and search the smashed cage for a unicorn horn and go into the cave to the north. Following the path until you get to some zombies wandering around, keep going further into the cave until you come to some knights; Harry, Carl and Jerro. Talk with them and you will get some food, and a few potions. After that, kill them all and get their three crests (red, green and blue).
After you have the crests, go further in taking the north-western path, using your plank on the flat rocks. You should come to another well, search this and then use the three crests and the unicorn horn with it. Now go through the large doors, and you will enter a very large area. This is one big maze, and you will most likely lose a lot of health here if your agility isn't high enough.
Going south, you should pass three bridges leading into the maze, remember these for later. When you're all the way south, you should see a staircase along the southern wall (you need to go a bit west), go down, and you will end up near a house, this is the Dwarven Camp. There are three dwarves, talk with Kamen for some food, and then talk with Nilhoof.
Nilhoof will offer you some Dwarven brew, it will hit you a bit, but Nilhoof will give you some food as well. Talk with Kamen again, and he will tell you about a witch that holds the secret to kill the evil Iban. Once he's told you about that, go back up the staircase and over the first bridge on the north-eastern side. If you fail any of the balance bridges you will fall down in to the underground, and you'll have to start over again.
When you fall, you'll end up near a bunch of soulesses, just walk south from there, and you should end up at the staircase near the Dwarven Camp. Also, you will lose a lot of health when falling down, so try not to do it too often.
Did you notice the house in the south-eastern corner of the maze? That's where you need to go. Once you're there, search the window, and you should see a witch inside saying something like "Here Kitty, Kitty". Do not try to open or knock at the door. Now you need to go find the witches cat. Go north-west in the maze, and you should shortly see a yellow dot on your map, that's her cat. Pick it up, and return to the witches' house.
Use the cat on the door, and you will automatically hide behind the corner. Now you can safely open the door, and search the chest inside, you'll find a stat restore potion and a super attack potion, an old journal (the history of Iban), and most important; a Doll of Iban. Now you'll need to do some demon fighting. Go north of the house, then west, and then south. You should see a level 91 demon called Holthion, kill him and get the amulet he drops, then kill Othainan and do the same, then Doomion. They all have 90 hits, and aren't really too hard to kill. When you have all three amulets, go to the chest north of Doomion.
Open the chest and you will get the Shadow of Iban, use this with your doll. After that you can head back down to the Dwarven Camp and talk with Klank. He will give you his gauntlets (very good gloves for Melee), and a tinderbox (keep it). Put the gloves on, and go in to the bigger of the two houses. Pick up a bucket and head over to the other house. Use your bucket on the barrel in there.
Now head east of the Camp until you reach a Tomb (it's Iban's, surprise, surprise). Use your dwarf brew on it, and light it up with your tinderbox. Now collect the ashes, and use these with your Doll of Iban. Head north and into the spider pit. Here you'll find the huge spider named Kalrag (level 78). Kill it, and you will automatically collect and use the blood with your Iban Doll.
This can be a pretty creepy place, and if you don't like spiders, I'd suggest you play on low detail, and have your camera angle as high as possible.
After that, go back up to the maze, and go north-east until you reach the most northern bridge into the maze. You will come to some level 1 creatures, Half-soulesses. Make sure you're wearing Klank's gauntlets, and then search all of the cages until you find the Skeleton of a Dove. Use this with your Doll of Iban.
Now it's time to beat this quest, you should now have a doll with a shadow, ashes, blood and dove; a Mini-Iban. Go back out, and south to the middle bridge, cross the few bridges, and make your way south until you see some level 13 Iban Disciples, kill one and you should have a full set of Zamorak Robes.
You should see some staircases right about here, so get yourself ready for some fun. You may only wield the Robes of Zamorak for this part. No armour, weapon, ring, amulet etc. When you're ready, open the doors, (Protect from magic extremely useful here. Put it on, and you won't have any trouble) and make sure you have your inventory open as well, since you'll need to eat a lot here. What you need to do is use your finished doll with the well in the room, while trying not to get killed by the spells coming up from the floor (looks like roots).
Once you've used your doll on the well, Lord Iban will die, and you will be teleported to another room. If you have a look at your inventory, you'll notice the Staff of Iban, 15 death runes and 30 fire runes. You will also see Koftic in the room, talk with him, and you'll see that he's sane again. After talking with Koftic, go out, and back to King Lathas in East Ardougne and the quest is complete.