Time to put on that Green Beret, stock up on supplies and go to town on a Sixth House base. Well, almost. Cosades wants you to have a reasonable chance at survival (a dead Nerevarine doesn't do anyone any good), so if you are not at least 6th level, he will suggest some training and general skill improvement and will give you 400 gold to do it with.
If you are at least 6th level, he will direct you to Raesa Pullia at Fort Buckmoth (it's directly south of Ald'ruhn) to get the information on the base's location and what to expect. Raesa is the lady in armor directly in front of you when you come through the front door. She doesn't have the most precise directions you have ever heard, but she's better than some. In short, she tells you to go to Gnaar Mok and ask the locals. Do so and you'll be told that it's somewhere north of Kartag Point (which is a short distance northwest of Gnaar Mok).
The entrance to Ilunibi is a bit hard to find, so pay attention to the little map in the corner. The entrance will show on that before it will show on your overland map.
The base is built in several sections. Rather than try to give directions, here are maps from the construction set. The five sections are Carcass of the Saint, Marowak's Spine, Tainted Marrow, Blackened Heart and Soul's Rattle.
Aside from the three creatures, you'll find a Fortify Magicka potion on the bottom of the pool in front of the waterfalls and you'll find a dead body near the Dreamer that has some leveled non-magical armor and a leveled enchanted weapon.
Just like the spine more or less connects all parts of the body, this spine more or less connects all parts of the base. Aside from what's marked on the map, there is nothing else in this section.
Aside from being thorough, there is really no reason to come through here. The dead adventurer has some leveled lockpicks and probes, some non-enchanted armor and an enchanted weapon. But unless you're very high level, it's probably not going to be any better than what you already have.
Aside from the various creatures, there is nothing else of interest in this area.
Dagoth Gares is about the only difficult creature to deal with in this section. As soon as you get within 1000 units of his location (that's about the entrance of his little cavern), you will get a greeting and an offer to join Dagoth Ur. It does not matter how the dialogue goes, he will attack you. As soon as you kill him, you will be infected with Corprus Disease (you can't avoid it). Aside from that, he's just an Ash Ghoul with a bit more fatigue and fewer spells than normal. This is not to say that this will not be a tough fight (because it will). But take him out.
As a reward for your determination in accomplishing this mission, you'll find a lot of nice goodies laying around his chamber. Be sure to look under the western Treasure Trough to get the gauntlets (+20 points to Strength and Agility - constant effect). There is also a 6th House Bell Hammer up on the platform. Makes a nice Warhammer and it's worth 5000 gold. It's not enchanted, so it won't hit creatures that have normal weapon resistances, but it's still worth a chunk of change and does 1-50 to normal critters.
When you report back to Caius Cosades, he will have canvassed his informants and come up with a possible cure for your Corprus Disease. That's Caius. Always planning ahead.