Sepulcher of Knowledge

The Sepulcher of Knowledge is a library tower located at the northern end of a fungal valley in north-northwest Maldraxxus. It stores the military writings of the greatest tactical minds to have ever served among the Necrolords. After the disappearance of the Primus, the Sepulcher was magically sealed. The Primus placed five guardians in the area around the facility, each connected to a runestone and embodying the virtues of one of the five houses of Maldraxxus, and the door was sealed with an elaborate magical lock controlled by a pair of obelisks that will only open the door after being fed with anima.

The valley surrounding the Sepulcher has been turned into a swamp by the slime flowing from Akarek's Tears and has become infested with devourers attracted to the remnants of the power in the facility. In addition to entrances from the south and northeast, the area is also connected via mountain tunnels to Spider's Watch in the west and the Forgotten Wounds in the east. The broker Ta'eran, located next to the entrance of the Spider's Watch tunnel, enlists adventurers to help him gain access to the tower and the writings within.

The Anima Conductor point to the House of Eyes for the Necrolord Covenant is located here.

Valley inhabitants
 

In the northeast, near the border with Akarek's Tears:

In the southwestern valley entrance:  Roaming near the Runestone of Constructs:

Next to the Spider's Watch tunnel:

At the five runestones during Repeat After Me:  

The tower
The interior of the Sepulcher proper only becomes accessible after opening the tower door during Slaylines. The tower's floors are initially protected by various traps and defenses: Tomes of War, Defense Protocol beams, Spinning Bookshelves, and Library Traps. The latter two despawn or become inactive after completion of A Complete Set.

The tower contains the following books:     
 * Alver's Annals of Strategy
 * Beginner's Guide to Polearms
 * Guide To Marching
 * How Not To Lose
 * Krexus's Guide To War
 * The Necronom-i-nom
 * Pretend To Win
 * War, Is It Art?
 * World of Crafting War

The top floor is inhabited by the Archivist and supposedly holds the tower's rarest volumes. However, upon defeating the Archivist, Ta'eran and the player discover that this is not the case and that the location of the rare books is actually hinted at by the Archivist's last words: "Victory only belongs to those brave enough to fall."

The hidden library
Below the valley floor is a hidden library storing the Volumes of Victory, the most important of all of the Sepulcher's writings. The chamber first becomes accessible during Trust Fall when Ta'eran and the player jump off the top of the tower and are teleported into the room. Players can subsequently use the Library Teleporters to travel between the chamber and the surface.

The chamber contains the following Volumes of Victory:     
 * Volumes of Victory: Vol 1
 * Volumes of Victory: Vol 38
 * Volumes of Victory: Vol 49
 * Volumes of Victory: Vol 52
 * Volumes of Victory: Vol 98
 * Volumes of Victory: Vol 102
 * Volumes of Victory: Vol 583
 * Additional Volumes
 * Additional Volumes of Victory

Speculation
The Primus' warning in The Door to the Unknown mentions that "Zovaal must not reach the sepulcher". In one of the random conversations between Opulence Ba'hare and Emissary Ba'solo, the latter begins to ask about "the location of the sepul--" before Ba'hare cuts them off with the words "Do not speak of the grand design!" It's unknown if these refer to the Sepulcher of Knowledge or something else.