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The Battlespire was the proving ground and testing facility for the Shadow Legion, the Imperial Battlemages, in the Third Era. This war college was (and it's ruins remain) located in a Slipstream Realm between Oblivion and Mundus, as such a realm allowed mortals to experience and comprehend Oblivion without being driven to madness. More simply, it was possible access numerous realms of Oblivion from the Spire through the use of teleportals and voidgates. The facility was moored in place by five great Anchors, which assured its safety and stability in the surrounding flux. Access to the edifice was granted via the Pillar of Light, a teleportation device linked to the Weir Gate.
During the Imperial Simulacrum, the Impostor Jagar Tharn betrayed the Battlespire to the legions of Daedric Prince Mehrunes Dagon. A pair of trainees became trapped inside; one was captured by Lord Dagon, but the other apprentice followed the trail through numerous Daedric gates to Dagon's citadel, where, by a subtle trick, they cast the Daedra Lord into Oblivion. Having lost Lord Dagon's support, Jagar Tharn's plot to supplant the true Emperor was doomed to fail.
The Weir Gate
The Weir Gate is the portal through which the Battlespire was accessed from Nirn, and which also provided a means of return. It's existence was known as far back as the Second Era, though it is unclear if the Battlespire as a whole existed at that time.
During the attack on the spire by Mehrunes Dagon's legions, the gate was sealed with a powerful sigil in the shape of the Daedric letter "Hefhed". This meant that no reinforcements could enter the Battlespire from Tamriel, and the battlemages already within were trapped.
The Star Galley Crib
Aside from the Weir Gate itself, the only access to the colleges was via the Star Galley, a magic boat which could navigate the surrounding environs. The vessel was constructed by Battlemage and Artificer Clarentavious Valisious, and was kept in the Star Galley Crib near the Weir Gate, though access was normally restricted.
This section of the facility is notable for being the home of the gigantic Mock Turtle, a member of the rare species of Great Turtle from which is made the famous Mock Turtle Soup, the Soup of Kings. He was known to have "dwell[ed] beyond the Weir Gate" in the Second Era.
The High Halls and Librarium
The upper sections of the Battlespire consisted of the war college itself. Here were found dormitories, mess halls, libraries, and more. Of particular importance was the voidgate. This device, powered by artifacts known as voidguides, allowed passage to Oblivion. The Soul Cairn was accessed in this manner during Mehrunes Dagon's invasion, but it is not known which other realms the voidgate was able to reach.
Several realms of Oblivion were known to be accessible from the Battlespire or from each other. These include the Soul Cairn, Shade Perilous, the Chimera of Desolation, the Havoc Wellhead, and Dagon's Hunting Lodge in the Deadlands.
The Invasion of the Battlespire
"...for had not Jagar Tharn been robbed of the support of the archfiend Mehrunes Dagon and his fell minions, then Ria Silmane and her champion might never have revealed Tharn's imposture, and long might the rightful emperor have languished imprisoned in Dagon's dark prisons within the Void." — from The Chronicles of Janisiere
During the Imperial Simulacrum in 3E 398, Battlespire was invaded by legions of Daedra in the service of Daedric Prince Mehrunes Dagon.[UOL 1] Imperial Battlemage Jagar Tharn, posing as Emperor Uriel Septim VII, made a deal with Lord Dagon in order to eliminate his enemies.
Mactana Greenway - Gatekeeper
Star Galley Crib Opening Mechanism
Operation without Permission of His Honor, The Venerable Artificer, is Expressly Forbidden
Gaze Upon this Device and its Subtle Workings, and Tremble with Wonder!
By the grace and gifts of Clarentavious Valisious, thus the Star Galley is Made Fast and Secured Thereby.
lomegan mariel surrendered portal keys, was struck down.
Sirran Angada - tharn henchman, disguised as Lomegan Mariel.
Paxti Bittor was the traitor. Master of teleportation and voidgates.
The Impostor Jagar Tharn betrayed the Battlespire, the war college of the Imperial Battlemages, to the legions of Daedra Lord Mehrunes Dagon. Two trainees trapped in the Battlespire escaped by fighting their way through daedric gates until they came to the citadel of Lord Dagon, where, by a subtle trick, they cast the Daedra Lord into Oblivion. Having lost Lord Dagon's support, Jagar Tharn's plot to supplant the true Emperor was doomed to fail. 
The invaders were known to wield Daedric Crescents, and succeeded in slaughtering most of those inside. The Empire later reclaimed the ruined Battlespire, and gathered up all the Daedric Crescents to be destroyed, though one is rumored to have survived.
Communications have been lost with the Battlespire, the Imperial Battle College's elite training center in a pocket realm in the vast chaotic emptiness of Oblivion. An agent, an acquaintance, is sent to investigate. She doesn't report back. So they send you.
You enter to find yourself trapped, your return through the Weir Gate blocked by a magical Daedric Sigil. The staff of the Battlespire are all dead, and the facility is crawling with Daedra. In addition, four of the arcane anchors holding the Battlespire in place have been disconnected. You must restore the anchors, or the Battlespire will be destroyed.
You can't go back to Tamriel. Once you have restored a teleport device, you can only search deeper into the occupied Battlespire laboratories and administrative center.
Fortunately, the agent who preceded you has left you reports of all she has learned. She has gone on ahead to scout and track down the traitor who betrayed the Battlespire.
From your agent's notes, you discover that a human servant of the Daedra Lord Mehrunes Dagon has induced a Battlespire mage to turn traitor and open a gateway for an invasion of Daedra. Your only hope is to fight your way past the Daedric troops, guards, and captains, steal arcane keys from the bodies of your enemies, and pass through the Beachhead Portal the Daedra used to invade the Battlespire.
This will lead you directly into the enemy's camps and routes of march -- into the myriad pocket worlds that float in the voids of Oblivion. At best, you might find a way back. At worst, you can avenge yourself on the Daedra invaders and the traitor who opened the way into the Battlespire.
Through the Portal you find yourself in the Soul Cairn -- a Daedric realm occupied by lost souls. You must fight your way past the maddened revenants and the Daedra garrison troops to make your way to the next pocket world -- the Shade Perilous.
In the Shade Perilous, you discover that a number of Daedric realms have been invaded and occupied to provide travel corridors from Mehrunes Dagon's invading army base to the Battlespire. To pass through to the next Daedric realm along the invasion path, you need the help of two Daedra lovers, Jaciel Morgen and Deyanira Katrece, victims of the invading Daedric army. Only Deyanira Katrece's tragic sacrifice can shake Jaciel Morgen from her despair enough to help you teleport to your next objective -- the Chimera of Desolation. Jaciel Morgen also promises to come at your call when you reach Mehrunes Dagon; she pledges to help you take your revenge on him.
The Chimera of Desolation appears to be a great island floating in the void. Here you find yourself the target of a Wild Hunt. Only with the help of the old mad hermit of the island can you find the Armor of the Savior's Hide and the Spear of Bitter Mercy to turn the tables on the Daedra huntmaster and his Daedric dogs chasing you. On the dead huntmaster you find the key to the gate that leads you to the next realm.
The Havoc Wellhead is the staging area for the Daedric invasion of Battlespire. Here the ambitious and competing rival captains of Mehrunes Dagon's armies are mustered with their respective troops, preparing to use the Battlespire as a bridgehead to invade Tamriel. You must play the rivals off against one another to get the keys to the next realm. You also get the opportunity to kill the mortal sorcerer who treacherously helped Mehrunes Dagon stage his invasion of the Battlespire.
The next and final realm is full of giant stone islands floating above a sea of lava. The floating stone islands are connected by flimsy bridges and guarded by mighty Daedra warriors. In the center of the realm is the Hunting Lodge of Mehrunes Dagon. You must enter the Hunting Lodge and confront Mehrunes Dagon, using the lore you have wheedled from Daedric informants, wearing the Armor of the Savior's Hide, and striking Mehrunes Dagon with the Broadsword of the Moon River (taken from the body of one of the Daedra Lord's most fierce and faithful bodyguards).
You call on Jaciel Morgen. She comes and distracts Mehrunes Dagon long enough for you to strike him, hard and true. The Daedra Lord is cast far out into the trackless void of Oblivion, and will be a long time finding his way back to his realm and his minions. You rescue your fellow agent held captive here, then return to Tamriel to receive the grateful praise of your emperor. 
þ"...and the Emperor then commanded to bring forth the two champions who had served him so well, who knelt before him and received his blessing. He bid them rise and said to them, '%APF, and %PCF, let all present know how these two have served their Empire, for had not Jagar Tharn been robbed of the support of the archfiend Mehrunes Dagon and his fell minions, then Ria Silmane and her champion might never have revealed Tharn's imposture, and long might the rightful emperor have languished imprisoned in Dagon's dark prisons within the Void.' And he then made them great presents of land and chattels, and raised them up among his counselors, and conferred upon them offices of great importance in his Legions."
þ"Reinvestment of Battlespire is neither feasible nor desirable at this time. The open provocation and sweet temptation this installation presents to the Daedric Powers is obvious from recent events, and the installation is of no significant value to the maintenance of the Pax Imperia. In light of recent turbulence in the Border Provinces, and considering more pressing demands upon the treasury, we recommend that the gates to Battlespire be sealed indefinitely."
þ"The Emperor questioned %PCF closely. %PCF said that, when struck by the enchanted sword, Dagon appeared to have been sucked away and compressed into an ever-smaller volume, and then to have disappeared. The Emperor then inquired among his counselors how long we might be free of the Prince of Destruction. Their answers were various, hopeful, and unpersuasive. 'You have done all that might be done, %PCF,' sighed the Emperor, 'but Oblivion cannot long hold Dagon, and when he returns, he surely will count us among his most-cherished enemies.'"
- According to The Doors of Oblivion, years after Dagon's invasion was defeated, the mage Morian Zenas and his apprentice attempted to use the Weir Gate to enter Oblivion. They found that although the gate still stood and the shattered academy remained accessible from Mundus, the link to Oblivion had been broken. However, this is contradicted by the Crafting Motifs series, according to which the two lived in the Second Era.
- Battlespire opening cinematic
- Events of Battlespire
- Elderscrolls.com Codex Archive
- The Doors of Oblivion — Seif-ij Hidja
- ESO Loremaster's Archive — Lord Fa-Nuit-Hen and Tutor Riparius Answer Your Questions
- Great Anchor warning plaque in Battlespire
- Scute of the Mock Turtle item description in ESO
- Letters for the Battlespire Hero — Vatasha Trenelle or Josian Kaid
- Scamp's dialogue in Battlespire
- Letter to the Grand Marshall of the Imperial Legion — Clarentavious Valisious
- Star Galley Crib Note
- The Mock Turtle's dialogue in Battlespire
- Tamrielic Lore — Yagrum Bagarn
- The Elder Scrolls - 10th Anniversary archive
Note: the following references are not from official sources. They are included to provide a rounder background to this article, but may not reflect established lore.
- The official timeline, archived from Jan. 25, 1998