Councillor Vay Hek is the Grineer boss on Earth, and can be found at Oro, which requires Mastery Rank 5 or higher to access.
Lotus: "Councillor Vay Hek rose to power by 'eliminating' his enemies on the Grineer council. He now has near-complete control which makes him one of the most influential Grineer."
Lotus: "Vay Hek's plans for complete System domination make him too dangerous. You know what to do."
Lotus: "Our operatives tell us Vay Hek has undergone an expensive regime of combat enhancement. Expect the worst."
Shortly after the Tenno begins progressing through the Assassination mission, a brief cutscene will play that shows Councillor Vay Hek's face as he starts to cackle in a long, drawn-out fashion.
Councillor Vay Hek: [laughs, then laughs again louder]
[Vay Hek's mask closes over his face as he turns around and flies away in his yellow robotic frame.]
Councillor Vay Hek: [laughs even louder, now slightly muffled by his mask]
Vay Hek's laughter will continue even after the cutscene ends.
Soon, the Tenno will arrive in a clearing where Vay Hek is hovering, surrounded by Grineer.
Councillor Vay Hek: "These criminals think they are committing a justice. Not in my court!"
Vay Hek and the Grineer will attack the Tenno. Vay Hek can only be damaged on his exposed face. After the Tenno has inflicted a sufficient amount of damage to him, Hek will fly away.
(when leaving) Councillor Vay Hek: "Grineer brothers! I've done my part. Now clean up this mess."
Vay Hek will confront the Tenno two more times. Each successive time he encounters them, he will start to use a new attack. Most of his abilities involve buffing allies or hampering the Tenno.
(upon the Tenno's arrival, second time) Councillor Vay Hek: "Why are these fools still breathing my air? This is unacceptable!"
(when leaving) Councillor Vay Hek: "I am too important for this nonsense. Finish them off."
(upon the Tenno's arrival, third time) Councillor Vay Hek: "Must I do everything myself?"
(when leaving) Councillor Vay Hek: "Do not think we are done here, Tenno!"
As the Tenno head to Hek's lair, he will taunt them once more.
Councillor Vay Hek: "I knew this day would come. I've prepared, changed myself. I've made sacrifices, Tenno! Are you ready for a sacrifice?"
The final stage is accessed by entering a broken pipe and sliding through it until it empties out into the arena. The water flow will prevent the Tenno from exiting via the pipeline until the fight is over.
(upon the Tenno's arrival) Councillor Vay Hek: "Now we finish this business. Once and for all!"
The arena consists of a few broken-down Grineer facilities surrounding a narrow chasm, into which flow several streams of water. High cliffs and aging metal fencing form tall barriers, keeping the Tenno inside. Grineer will emerge from the buildings to attack as Vay Hek engages the Tenno in his robotic frame. Upon reaching low health, a short cutscene will play.
Councillor Vay Hek: "Ahhh!!! Curse you, Tenno. You [chuckles] got me! [laughs]"
[Vay Hek flies over to a large, yellow biped robot. His flying frame drops down on top of it, slotting into place and allowing him to control it. The robot – his Terra Frame – comes to life, its weapons readying as it emits a loud, birdlike screech.]
In his Terra Frame, Vay Hek no longer has specific weakpoints and can be damaged anywhere on his frame, but he has much stronger attacks. The Terra Frame's right arm wields a rocket launcher, while its left arm dispenses floating drones that broadcast propaganda or call down orbital strikes. He retains the use of the machine guns from his flying frame. He also has additional dialogue lines in this form.
"Tenno... what makes you think you are ready to fight Vay Hek?"
"You are not ready for this fight."
"Your Lotus is leading you to destruction."
"You Tenno are such a waste of perfectly good violence."
"This is too easy! Tell your Lotus to send more capable fighters next time."
"The first Grineer who brings me the head of a Tenno will be permitted to wear its skull as a helmet!"
"Kill. Kill. Kill. Kill the Tenno scum!"
When Vay Hek's health is depleted, his Terra Frame will begin sparking and jerking erratically. Vay Hek will eject in his smaller robot frame and fly away, as his Terra Frame explodes.
Lotus: "Thank you, Tenno. It should be a long time before we hear Vay Hek's ugly voice again. Now, get to extraction. You've earned it."
The pipeline that the Tenno used to enter the arena will no longer have flowing water, and it can be used to exit the area and head for extraction.
Councillor Vay Hek first became active in the affairs of the Tenno when they explored the Plains of Eidolon. In the Saya's Vigil quest, Hek serves as an antagonist, seeking the same thing as the Tenno: information on the fabled glass warrior Gara. Vay Hek is also involved in the recurring Plague Star and Ghoul Purge events that take place on the Plains. During the Cicero Crisis, he attempted to poison Earth's forests to make room for more Grineer living space and infrastructure.
Later, it was revealed that Hek was rebuilding the shattered Fomorian fleet, under the name Project Tethra. During Operation: Tethra’s Doom, the Tenno interfered in Fomorian construction by discovering the location of Hek's shipyards and sabotaging the Fomorian power cores. This effort was successful enough that Vay Hek was forced into hiding, and could only be found by tracking down his elite Prosecutors on Ceres. However, his efforts to build the Fomorian fleet continued, and the Tenno sought tactical parity via an Orokin-era flight platform known as Archwing. In Operation: Gate Crash, Vay Hek invaded Orokin Towers in the Void in order to preemptively destroy records of Archwing technology, but the Tenno ultimately developed and repurposed the platform in the Archwing quest. Operation: Eyes of Blight saw Hek's armada of Balor Fomorians finally unleashed, and he destroyed most of the Tenno Relays. Using their Archwings, the Tenno inflicted massive casualties on the fleet, and Hek subsequently fell out of the Queens' favour. In the Law of Retribution Trial, the Tenno defeated Vay Hek once and for all as revenge for the destroyed Relays. Many of Vay Hek's activities are covered in the Timeline Discussion about Project Tethra.
Councillor Vay Hek also played a role in The New War. He is mentioned in the Ogma Elite Tactical Alert, by the Grineer propagandist overheard on the Orbiter's Radio Scanner, in the Codex entries of the Devourer Ghoul, Vay Molta and the Grustrag Three, and by Cephalon Cordylon, Kahl, and Horrek. The Ascension webcomic reveals some of his past. Vay Hek is also the Grineer leader who sends the Grustrag Three after Tenno who support the Corpus in Invasion missions.