Sekrion, Nexus Mind
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Sekrion, Nexus Mind was a massive Vex Hydra that lives deep within Venus. It is the Axis Mind in charge of converting Venus into a Machine World and served as the final boss of "The Nexus" Strike. In The Dawning, Sekrion would return as Sekrion, Subverted Mind.
Sekrion oversees the Vex's projects involving the Nexus, that converts the planet into an expansion of the Vex network from beneath the crust using its impressive computational capacity to coordinate between Minotaurs operating in the construction role. However, a fireteam of Guardians was dispatched by Ikora Rey to end the Axis Mind and hamper the Vex effort to incorporate the entire planet into their network. 
Sekrion's loss brought a major setback to the Vex on Venus as the planet's conversion is halted, prompting Theosyion, the Restorative Mind to bridge time and bringing the Vex Mind back from the past to resume its duties. Its efforts were however uncovered by Ikora and the Guardians sent to destroy Theosyion, preventing any alternate outcome that would replace Sekrion's destruction. 
The Vex still continue to struggle since their numerous losses to the various forces of the Solar System, namely the Guardians and the Taken. To counter the Guardians and restart the process of converting Venus into a machine world, the Vex uses its ontological technology found in the Vault of Glass to repair the Nexus Mind. Cayde-6 refers to this plan being a fearful, desperate plan for the Vex.
However, Ikora Rey's Hidden intelligence discovers this plan and so organizes the Guardians to destroy it before it can complete its work, all while dealing with the remaining Taken on Venus. After being guided by Ikora and Cayde-6, the Guardians reach the restored Nexus Mind, which was corrupted by the Taken as Sekrion, Subverted Mind. Despite its restored chassis and newly upgraded abilities, the Taken Vex Mind is destroyed once again.
Sekrion is the final boss of The Nexus strike. It fights like an ordinary Hydra, but fires its Aeon Maul more rapidly. It stays in the center of the arena, allowing Guardians to avoid its fire by ducking behind stone pillars. Additional Vex will spawn to flush out the players around the room, so players should be wary to not completely focus on the boss.
The Nexus: Revisited
In the Revisited version of the strike, Sekrion makes a return as the final boss. Once again, it stays in the center of the arena, pounding Guardians from afar with its Aeon Maul. It has gained a special mechanic: like the Oracles, if you get Marked by the Void then every Guardian must step in a pool of Light to remove the debuff, or else die after your screen goes completely black. Alternatively, in the Taken version of this strike has players remove Sekrion's shield by firing an Aegis at him so Guardians can defeat him.
Sekrion, Subverted Mind
As with any Taken Hydra, Sekrion will have no shield by default, leaving it vulnerable to attack. Just like its Vex counterpart, it will stay in the center of the arena and open fire on Guardians using its Aeon Maul. However, every so often, Sekrion will activate a barrier around itself that can only be taken down by the Super attack of the Aegis shield relic from the Vault of Glass, which will spawn in different spots along the edge of this arena. Applying the same mechanic used during the Templar encounter from the Vault of Glass, one player must pick up the Aegis shield to begin charging the relic's super, then blast Sekrion's barrier once it's at full charge. This will deactivate Sekrion's barrier, initiating a damage phase. The Aegis will immediately despawn from the players hands after each use and respawn in another random spot along the outer ring of the arena. Players will need to coordinate Aegis blasts and damage phases while maintaining control over waves of "lesser" enemies to put an end to Sekrion. As always, rinse and repeat.
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