You should not be able to get this quest until at least one day has passed since the end of the previous one.
Cosades now wants to know about the Ashlanders and their Nerevarine Cult, so he sends you off to Ald'ruhn in search of Hassour Zainsubani, an Ashlander who decided that life as a trader was better than life in the wilderness. Cosades gives you 100 golds to get Hassour a suitable gift.
You'll find him in the Ald Skar Inn, down in the room below the bar, but don't go talk to him just yet. There is no one to fight, but a little sweet-talking will go a long way with this gentleman. Cosades told you to get him a gift, so start asking around and you'll find that this particular Ashlander likes poetry. There are a few suitable books of poetry floating around:
"Words of the Wind"
"The Five Far Stars"
You can purchase one or two of them from the bookseller near the Manor District entrance. Or, if you have a more larcenous mindset, you can just steal one from his room and present it to him. Failing in all of that, you can just get his disposition up to 80 (bribes and flattery work pretty well with this guy). Anyway, once his disposition is up where it needs to be, he'll be happy to tell you all about the Ashlanders.
Actually, he won't tell you about them; he'll just give you a copy of his notes and urge you to read them. Follow every conversation thread that you can to learn about how to behave in an Ashlander camp. The information will be very useful shortly.
He will also let you ask him about his son, Hannat, who is exploring the depths of Mamaea. It's a not-so-subtle hint that you have another way of getting his disposition up if nothing else will work.
Once you have the notes, report back to Cosades. You'll get the real information behind your missions, a copy of the decoded package you delivered to Cosades a few missions ago, a promotion, 200 golds, instructions to pick up supplies from Somutis Vunnis and Crulius Pontanian at Fort Moonmoth, and orders to go talk to Sul-Matuul and Nibani Maesa at the Urshilaku Ashlander camp.
It would be wise to follow those instructions in that order.
The "essential supplies" you'll get from Somutis Vunnis (in the Imperial Cult Shrine) are:
The "essential supplies" you'll get from Crulius Pontanian (behind one of the tables) are:
Mamaea is a three-section Sixth House shrine located southwest of Maar Gan and directly west of Bal Isra (Indarys Manor if you have joined a Great House and have got to the stronghold construction/destruction quests). It will be on the other side of the BIG mountains (Levitate or really good Acrobatics skill is highly recommended). It has a several leveled creatures in it, about half being slightly above your level. The way that a leveled creature works is fairly straightforward.
When you enter the area, the game checks your level. In the case of Sixth House shrines and whatnot, it either adds 2 to it (if the creatures are supposed to be slightly tougher than you), adds nothing to it (if the creatures are supposed to be about your level), or subtracts 2 from it (if the creatures are supposed to be slightly weaker than you). It takes the resulting number and compares it to a list of possible creatures and will randomly grab one from the list and have it be there when the area finishes loading.
The list for this shrine (with levels) goes like this:
The sections use a few different lists. The Corprus creatures are on half the lists and not on the other half.
So, if you are 12th level and enter the area, the game will randomly generate anything that has a level less than 12 (14 if it was a +2 list, or 10 if it was a -2 list). If none of the creatures on the list are less than or equal to the resulting level number, then nothing will appear. That probably isn't an option in this case, though it might be if you stumbled in here before starting the Main Quest. If that's the case, I'd advise stumbling back out -- quickly.
Four leveled creatures running amok down here, three of them being from the +2 lists, plus three Dreamers. Dreamers, if you haven't run into them before now, are 1st level Dark Elves who run around without any clothes and beat up on poor defenseless Neverarine wannabes who stick their noses into the wrong holes in the ground. In and of themselves Dreamers are little more than a nuisance, except that these can give you a disease called "Wither" that drains your STR and END 10 points.
Elsewhere in the area are a dead skeleton (dead dead, not un-dead) that has some random loot on him, 10 golds, and a door to the Sanctum of Black Hope.
Four leveled creatures (you might find a Corprus Stalker or a Lame Corprus in this bunch), four Dreamers, a Dreamer Guard (basically a 3rd level Dark Elf fighter-type), Hannat Zainsurbani (locked up -- the guard has the key), and Dagoth Araynys, a 30th level Ash Vampire:
Tough critter, yes? Do what you have to, but take this Dagoth out. Now would be good, but later will work if "later" is before you get to the end of the Main Quest. You won't know it from anything within the game, but each one of these ash vampires that you can kill now is supposed to weaken Dagoth Ur (doesn't work unless you do your own script fix - see WRFan's script on the Morrowind forums). Each Ash Vampire's death subtracts 5 points from Dagoth Ur's STR/WIL/SPD, 50 points from his Health/Fatigue, and 250 from his Magicka. Dagoth Ur's base stats are 100, base Health and Fatigue are 1000 and base Magicka is 5000 (script says 5000, stat sheet says 1000). Since there are seven Ash Vampires in the game, that means you can knock 35% off of Dagoth Ur before you ever get to him. If you don't get to all seven, they will die when you destroy Dagoth Ur (taking their treasures with them, most likely).
Anyway, there isn't anything else in the area except what the various critters are carrying on them. However, you will find a door to the Shrine of Pitted Dreams.
Freeing Hannat would be the prudent thing to do at this point. He will want you to escort him to the entrance. Once that's done, he tells you to go back and tell his father that he's OK and says that Hassour will reward you. When you report to Hassour about this, you'll get a Ring of Knuckle Luck (Cast When Used: damage Fatigue 3-120 on Touch), an Imperial Netch Blade (4-9/4-9/5-11 plus Cast When Strikes: damage Health 3-5 for 5 seconds) and 5 Raw Ebony.
Five leveled creatures (two of them at your level +2), five Dreamers, a Dreamer Priest (a 2nd level Dreamer with armor and a staff), a couple of dead skeletons (with their usual random loot), a few ingredients, a pair of Ebony Boots, a Daedric Helm, a Cephalopod Helm, a Standard Fortify Agility potion, and Fang (a dagger that does 20-40 Shock damage When Strikes). The really good loot is way back at the back of this section, so you'll have to plow through all of the section to get to it.
Now for the really bad news. All of the Dreamers will respawn and most of the leveled creatures SHOULD respawn. So if you're going to carry out all of this loot, you'd best do it AFTER you get Hannat to the front door. My recommendation would be to rescue Hannat, fight your way back here (running works, too) to the loot, gather it all and then teleport (Recall, Divine Intervention, Almsivi Intervention). You may not be able to move when you're over-encumbered, but you can always teleport to safety.