Knights of the Nine is an official plug-in released by Bethesda for Oblivion. KotN, as it is often abbreviated, is substantially larger than most of the other plug-ins. A list of frequently-asked questions and their answers can be found on the FAQs page.

A fallen King has been unchained from the darkness of Oblivion to seek vengeance upon the Gods who banished him. Only a champion pure of heart can vanquish the evil that has been released upon the land. You must heed the call, reclaim the lost relics of the Divine Crusader, and return the Nine to glory. New dungeons, characters, quests, and mysteries await

Unlike most of the official plug-ins, this plug-in adds multiple quests. Also, a quest is not immediately added to your list of current quests when you first install the plug-in. The new quests are:

Pilgrimage: Complete the Pilgrimage of the Nine Divines to show that you are worthy.
The Shrine of the Crusader: Retrieve the Helm of the Crusader from Pelinal's lost shrine.
Priory of the Nine: Retrieve the Cuirass of the Crusader from the Priory of the Nine. The following four quests must be completed before Priory of the Nine is considered complete:
Nature's Fury: Retrieve the Boots of the Crusader from the Shrine of Kynareth.
The Path of the Righteous: Retrieve the Mace of Zenithar from the Chapel of Zenithar in Leyawiin.
Wisdom of the Ages: Retrieve the Shield of the Crusader from Fort Bulwark.
Stendarr's Mercy: Retrieve the Gauntlets of the Crusader from the Chapel of Stendarr in Chorrol.
The Faithful Squire: Retrieve the Greaves of the Crusader from Lathon.
The Sword of the Crusader: Retrieve the Sword of the Crusader and reconsecrate it.
The Blessing of Talos: Obtain the ninth relic: The Blessing of Talos.
Umaril the Unfeathered: Defeat Umaril and kill him for good, in both the physical and spiritual planes.

Crusader Relics
Places
The following places will all appear in Tamriel after you install the plug-in. All of them can be discovered without starting any of the plug-in-related quests. However, access to the interiors may be prevented until the appropriate quest has been started.

Fort Bulwark — A medium fort northeast of Leyawiin containing conjurers (plugin-specific). (map)
Garlas Malatar — A medium Ayleid ruin northwest of Anvil containing Aurorans (plug-in specific). (map)
Grove of Trials — An outdoor area west of the Imperial City on the edge of the Great Forest containing one unique bear (plugin-specific). (map)
Kynareth's Grotto — A small cave west of the Imperial City (plugin-specific). (map)
Lord Vlindrel's Tower — A tower (no dungeon) southwest of Chorrol (plugin-specific). (map)
Priory of the Nine — A chapel and priory located southeast of Skingrad (plugin-specific). (map)
Shrine of Kynareth — An outdoors shrine west of the Imperial City (plugin-specific). (map)
Vanua — A medium Ayleid Ruin southeast of the Imperial City containing undead (plugin-specific). (map)
Also, some changes are made to existing places:

The Chapel of Anvil is ruined.
Gauntlets of the Crusader appear in a corner of the Chapel of Stendarr in Chorrol.
Lord Vlindrel's tomb and wraith are added to Underpall Cave.

Items
Several new items, many of them leveled, are added to the game with this plug-in. The new items page provides a complete list of these items and their statistics.


Magic
The new magical abilities and lesser powers added by this plug-in are detailed on the plug-in's magic page.


Factions
Two new factions are added with this plug-in:

Nine Divines

This faction actually existed before the plug-in, but previously could not be joined by the player
When you begin Pilgrimage, you are given the rank of Pilgrim in this faction
Membership in this faction allows you to freely enter all areas within chapels (including undercrofts and living quarters).
Knights of the Nine

As you progress through the quests, your rank in the Knights of the Nine faction changes.
At the start of The Shrine of the Crusader quest, you become Knight Errant.
After obtaining the Cuirass of the Crusader during the Priory of the Nine quest, you become Knight Commander.
At the start of the Umaril the Unfeathered quest, you become Divine Crusader.

Creatures and NPCs

Your Knights of the Nine
At different points, people will arrive at the Priory of the Nine to join.

Areldur, from the Chapel of Stendarr in Chorrol, is found reading and wants to join the Priory of the Nine.
Brothers Geimund & Gukimir are found sparring in the practice room and request to become Knights of the Nine.
Sir Thedret, who has escaped from Fort Bulwark, is found in the dining room and requests to become a Knight of the Nine.
Sergius Turrianus is found in the basement. He says he heard from his friend, Sir Thedret, that you would need a blacksmith. He will not join you in combat.
Avita Vesnia, from the shrine to Kynareth, is found in the sleeping area and asks to become a Knight of the Nine.
Lathon, a companion of Sir Roderic, is found in the entrance. He reveals some plot points, and requests to become a Knight of the Nine.
Brellin appears in the chapel and requests to become a Knight of the Nine.
Carodus Oholin, from the Chapel of Zenithar in Leyawiin, is found in the dining room and asks to become a Knight of the Nine.
If any of these knights die during the final quest or after Knights of the Nine has been completed, the named knight is replaced by an anonymous "Knight of the Nine". These knights are not as powerful as the named ones and do not cast magic.

After the questline is complete, you can ask any of your Knights to follow you, one at a time. There is a known bug caused by asking a knight to follow you before they have actually joined the faction; see the bugs section for details.


Other NPCs
Amragor
Aron Verethi
Beem-Kiurz
Eris Senim
Esbern
Inius Colus
Jesper
Kellen
Kinther
Pelinal Whitestrake
Selene Duronia
Sir Amiel
Sir Berich (Lord Vlindrel)
Sir Caius
Sir Casimir
Sir Gregory
Sir Henrik
Sir Juncan
Sir Ralvas
Sir Roderic
Sir Torolf
Stanet
The Prophet



Creatures
Aurorans
Umaril the Unfeathered
Forest Guardian
Lord Berich Vlindrel

Notes
This plug-in seemingly is intended for the Crusader class, or similar types with emphasis on heavy combat and restoration magic.
It's best to begin this plug-in with a new character, because the loot will last a lifetime (see below).
The character involved must be and remain a verified saint. Otherwise, you don't get to use the loot. On the other hand, your deeds will probably be remembered in the lore of future Elder Scrolls games.
If you complete this quest before getting the recommendation for Restoration Master Training, access to the Restoration Master Trainer Oleta will never become available as some key characters perish during this quest.
The whole point of the plug-in is to acquire a full set of armor and weapons (the Relics). These Relics have powers far beyond their base stats (be sure to read the Items section above). In summary:
Advantages of Relics
All are good items for their basic stats.
The Relics can be leveled up to keep pace with your character, so you really never need to replace them. Besides, this avoids the embarrassment of selling these holy artifacts once you've outgrown them. This is done on the display stand in the basement of the Priory of the Nine.
The display stand also fully repairs and recharges the Relics, so you don't have to spend any money on that, either.
The above features remove much of the money-grubbing pressure from your character, allowing you to just have fun without worrying about getting every last arrow back to town to sell.
The Relics give you a bunch of Lesser Powers that allow you to act like a real saint. You can buff and heal others at a ridiculously low magicka cost, for instance.
Completion of the quest line (as in acquiring all the relics) makes you a faction head with your own guild hall (Priory of the Nine) and private army of NPCs who will fight with you (1 at a time). Part of the deal is your own chapel with functioning altar. You also get a smith to repair non-Relic items, although he charges normal rates for this.
Wearing the Relics gives your character a huge boost in the restoration skill.
Disadvantages to the Relics
You can only equip the Relics if your Infamy is 1 or less. If your Infamy ever reaches 2 or more, you must repeat the Pilgrimage to a Wayshrine of every deity. This resets your Infamy to 0, allowing you to use the Relics again.
Items and effects that temporarily increase your Infamy have the same effect. For example, the relics will not equip when the Gray Cowl of Nocturnal is equipped.

Bugs
It is possible for one of the Knights of the Nine to be stuck permanently following your character if you ask them to follow you at the wrong point in time. Since the knights subject to this bug will also be essential when the bug is triggered, it is impossible to ever get rid of them. The bug occurs if you do the following:
Complete the entire quest line (up to the point where Umaril is killed) without recruiting all nine knights.
During Sir Thedret's speech at the end of the Umaril the Unfeathered quest, you approach an unrecruited knight, and at that time simultaneously recruit the knight and ask him to follow you.
To avoid this problem, be sure to recruit the knights either before or after Thedret's speech, and do not ask a knight to follow you immediately after recruiting them.
Once the bug has been triggered, it is not normally possible (other than using the Console) to make the NPC stop following you.
On Xbox 360, one option is to disable the entire Knights of the Nine plug-in, load your saved game and resave it, then re-enable the plug-in. Before doing this, make sure to remove any items stored in the Priory or other locations added by KotN. You will lose all items added by KotN, and you will need to redo the KotN questline from the start.
If you have not done the Daedra Quest Molag Bal then you can simply complete that quest while he is following you and he will disappear.
This bug is fixed by the Unofficial Official Mods Patch.
Sometimes a piece of the Crusader Relics will not appear on the stand when you put it on the stand for recharge and repair. If this happens, you can fix the problem by just dropping the item on the ground and picking it up again.