Eye of the Jailer

The Eye of the Jailer (or Jailer's Eye) is an enormous soul seeker used by the Jailer to observe his realm, the Maw. It serves as a boss in the Sanctum of Domination, and is also encountered during the Chains of Domination campaign.

Adventure Guide
''When Azeroth was young, Odyn traded his eye to Mueh'zala to peer into the Shadowlands. Vain Odyn was deceived, for the Jailer twisted the Eye to serve his own ends. The Eye's unwavering gaze allows the Jailer to see all that transpires within his domain--and perhaps others as well.''

The keeper's eye
The Eye of the Jailer was originally the left eye of Odyn, one of Azeroth's titanic keepers. In order to learn how to bring the souls of fallen vrykul from the Shadowlands to his own realm, the Halls of Valor, Odyn decided he needed to see into the world of death and contacted the great spirit Mueh'zala with the help of the sorceress Helya. The keeper plucked his own eye from his head and gave it to Mueh'zala, allowing him to see through it beyond the veil and into the Shadowlands. There he saw the winged kyrian of Bastion, inspiring him to create his own beings based on them: the Val'kyr. Unbeknownst to Odyn, Mueh'zala subsequently delivered the keeper's all-seeing eye to his master, the Jailer, who transformed it into a weapon of darkness.

The Eye constantly searches the Maw for enemies of the Jailer, and once an individual in the Maw has drawn the Eye's notice, anything of import they do will catch the attention of the Jailer, causing him to unleash increasingly deadly threats against them. However, if the victim hides from the Eye for long enough, the Jailer inevitably turns his attention elsewhere. Furthermore, one can conceal themself from the Eye using tools such as broker wards.

Chains of Domination
When the Jailer pulled the realm of Korthia into the Maw, his Eye physically accompanied the Mawsworn forces attacking the realm. However, the city's powerful magic prevented the Eye from breaching its defenses or peering inside.

In Desmotaeron, Maw Walkers discovered that the Jailer's servant Helya was capturing and sacrificing Odyn's Val'kyr. The heroes freed the Val'kyr Danica and flew with her to Skyhold, where they delivered Helya's message to Odyn: that she would twist every Val'kyr he sent and that his own eye betrayed him. Odyn declared that this "insult" could not stand and ordered the heroes to create an instrument, the Ilskahorn, with which they would lure his Eye to battle, and to recruit a group of worthy Val'kyr to accompany Danica in battle. The Maw Walkers returned to the Maw and, after binding the Ilskahorn to the Maw with the help of the broker Ve'nari, traveled to the terminus of the River of Souls and used the instrument to summon both the Val'kyr and the Eye. The heroes and Val'kyr managed to defeat the Eye's lesser minions and forced it to flee back to its master's side in Torghast. Both Odyn and the Maw Walkers' allies rejoiced at the Eye's defeat, emboldening their continued advance against the Jailer.

Strategy
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Trivia

 * The story of the Eye has been revealed gradually over the course of several expansions:
 * Chronicle Volume 1 (2016) briefly mentions that Odyn "studied the energies [of] the Shadowlands" and that the knowledge he gained allowed him to create the Val'kyr, but does not elaborate on how he did this.
 * The Legend of Odyn, an object made readable in patch 7.2 (2017), introduced the in-universe story of "The Keeper's Eye", which describes how Odyn conceived the idea of the Val'kyr after sacrificing his eye to an unnamed "great spirit" of the Shadowlands in order to see into the realms of death.
 * Following the announcement of World of Warcraft: Shadowlands at BlizzCon 2019, Steve Danuser revealed in an interview with The Lost Codex that the aforementioned "great spirit" was an ally of the Jailer, and that players would, at some point in the expansion, discover that there was a "specific reason" for why the Jailer wanted the eye of a keeper.
 * In a May 2020 interview with SA Gamer, senior game designer Johnny Cash revealed that Bastion and the kyrian was the main thing Odyn saw when he peered into the Shadowlands, and they served as his inspiration for the Val'kyr.
 * In the Other Side dungeon, Mueh'zala boasts about striking a deal for Odyn's eye, confirming that he was the "great spirit".
 * Though the Eye of the Jailer mechanic has been present in-game since the launch of Shadowlands, the Eye was only revealed to be a physical creature with the announcement of patch 9.1 at BlizzConline (February 2021). In the patch announcement, game designer Jeremy Feasel also mentioned that "Those of you who were fans of the lore of the titan keeper Odyn might have some interest in [the Eye of the Jailer fight]", hinting that the two were connected, which was later explicitly confirmed in-game by the Focusing the Eye chapter of the Chains of Domination campaign and the Adventure Guide entry for the Eye's boss encounter.