"I'm to travel to an obscure waypoint marked by Teav on the recovered Warlock Scroll. I should return to Norlene after a thorough investigation."
Primeval Hollow is near the border to Black Marsh, near the Knights of the Nine location, Fort Bulwark.
Norlene will also fill you in on some of Verus's findings from his studies. Apparently, the Spire builders are known as the Warlocks. They were half man, half beast, practiced occult magic, and were called "Warlock Demons" by the Ayleids, who apparently did not approve of them.
On the south-western side of this large complex is a door to the Lost Deity Shrine.
"I've found a Memorial Stele sitting next to a Vacant Shrine. I should return with it to Norlene."
Return to the Guild and speak to Norlene. Norlene translates the stele, although she doesn't take it from you, and says it's a memorial to Luminai. Apparently the Lost Deity Shrine is a Lost Shrine of Luminai, a vanished deity. This completes this quest.
Norlene will promote you to Fellow when asked, and for a reward you can have the key to Display Case K. Display Case K is in the Library and contains a Dwemer Wing Schematic. Norlene will also sell you a new poster, Sinister Glyph Index, and Domitian Verus will now sell you Claw Poster.
The glass cover of the pedastal holding the Carved Vase on the right hand balcony is now removed.
"Sova has been asking for me. Perhaps she's decyphered that Ayleid Scroll I brought her earlier?"
The sewers themselves may have been previously visited in the Main Quest sub-quest The Path of Dawn. The entrance to the Ayleid ruins is in the sewers to the south east of the Elven Gardens sewer entrance, and may well have been passed during The Path of Dawn if The Lost Spires was already installed when that quest was done.
"I've found an Ayelid doorway into what must be the labyrinth ruin. I should explore it."
The Lost Ayleid Ruin itself is infested with undead.
There are three different sections to this ruin. The first is just filled with dangerous undead, and you need to make your way through it to open the way into the second part.
In the second part of the complex, the Substructure, you need to get to a large room in the south east with three pairs of minotaur statues and the skeletons of three fallen adventurers on the floor.
Lord Pratal is extremely dangerous and can heal himself once when at low health saying as he does "Blood heals me." The three fallen adventurers have healing and sorcery potions which should definitely be taken. The safest way of beating Pratal is running into the area he came from, picking the lock to Pratal's Coffer and taking Lord Pratal's Key, then running to one of the doors and leaving. Lord Pratal is too big to easily fit through the doors, although he can glitch through some of them. There is a Lockpick on one of the fallen adventurers and three in the Ayleid Cask at the far end. If you leave by the door that was originally locked when you entered, it may be easiest to not bother with Pratal at all, although you could snipe him with Destruction spells.
Another notable item in Pratal's Coffer is the Ayleid Cleansing Shrine which will cure all your afflictions once per day.
The final section of the ruin, the Labyrinth is, not suprisingly, labyrinthine. The main part is a maze of 72 circular rooms in an 8x9 grid, each of which can have up to four exits. The local map is absolutely no use here, as the exits cannot be made out on it. All but one of the dead ends has a Fallen Sentinel guarding it.
Make your way through the maze to the eastern side which leads to a large room with shallow water and a pyramidal structure in the centre. There is a bridge over the water to the structure, but it collapses when neared, so it will be necessary to jump the gap. At the top of the structure is the sarcophagus of Aeon IV.
"On top of Aeon's Sarcophagus lay this large stele. I should return with it to Sova."
At the back of the centre structure is a button which, when pressed, causes two flight of stairs to raise up, allowing access to an exit that will take you back near the entrance of the first ruin, allowing the Lost Labyrinth to be exited much faster. This exit requires jumping down from an overhead catwalk, so cannot be used the other way.
Take the Stele back to Sova at the Guild. She initially takes it from you and, when asked, says it describes a victory of the forces of King Aeon IV over a "twisted evil." It is not known what the twisted evil is, but it has been mentioned before in Solan's research in reference to the lost deity Luminai. Sova says that there is a map on the Stele that probably depicts where the battle was, and gives you it back so you can take it to Aster Cei to identify the location. There should be something on the battleground that will help identify who Aeon was fighting.
When asked about a reward, she gives you the key to Display Case I, which is up the left hand stairs at the far end of the balcony and contains a Dwemer Cylinder, a Dwemer Tube and a Dwemer Piston.
The glass cover to the pedastal holding a Dragon Egg to the right of the door to the Library is now removed.
"Aster Cei was able to identify the the landmark depicted on the Victory Stele as the Panther River. The battle-point shouldn't be hard to locate now."
"A new Spire! Then it must have been the Warlocks against whom Aeon fought. They are the "twisted evil." I should explore the Spire."
The Beseiged Spire Ruin itself is now occupied by Albino Goblins.
"This Spire is decimated from within. Obviously the great battle depicted on the Victory Stele took place here. I should tell Sova of this."
The Portal is still operational when activated. At this point, you can exit the Spire and return to Sova, as the requirements have been fulfilled, or you can continue clearing out the Spire and go through the Celestial Portal.
When you return to Sova you tell her that you found a third Spire. Sova speculates that the Ayleids wipes out the Warlocks in retribution for the loss of Luminai. This completes the quest and she promotes you to Expert. She also tells you that Rythor wishes to speak to you.
The reward you get if you ask for it is the Key to Display Case G, which is up the right hand stairs at the far end of the balcony past Rythor's Quarters and contains a Dwemer Machine Schematic. Sova will now sell you a Dragon Poster and Aster Cei a Fish Poster.
The glass cover to the pedastal in the Library holding a Patterned Vase is now removed.
You now need to go speak to Rythor about the Mining Tunnel to start the quest Finding the Horn.