The Grustrag Three

 The Grustrag Three is a group founded by Councilor Vay Hek, which comes after a Tenno who has completed at least one Invasion mission against the Grineer. The Grustrag Three were formerly regular Grineer marines who (while aboard a troop transport) suddenly slaughtered their fellow comrades, claiming that they were growing "impatient for the killing". After being committed to the Grustrag Reconditioning Facility, Facility Director Tengus discovered that the marines suffer from a rare mutation brought about by cloning, vastly increasing their combat potential at the cost of deteriorating mental faculties.

Under Vay Hek's order, Tengus reconditioned them, with the intent of focusing their aggression towards the Tenno instead. Although mostly successful, the Three still had lingering violent behavior towards fellow Grineer. To counteract this, Tengus developed the Grustrag Bolt, a device that limited their effectiveness against other Grineer personnel. Tengus had additional Bolts created, to be used against the Tenno during the Three's operations.

With The Grustrag Three now prepared for combat, Vay Hek deployed them with the sole purpose of attacking those who supported enemies of the Grineer. However, Sargas Ruk criticized Hek's actions, reminding Hek of the soldiers The Three have killed and warning him that they would eventually grow out of his control.

Leekter= The Carabus Sentinel will accompany each of the three Grustrag Three members. They do not have a name in game, only a health/shields bar, similar to Loki's Decoy. The sentinels are equipped with Laser Rifles.
 * -|Shik Tal=
 * -|Vem Tabook=
 * -|Carabus Sentinel=

When a player is defeated by the Grustrag Three, one of the three sentinels will detach from its host, then hover and deploy a shield over the downed player. It prevents other teammates from reviving the player while the sentinel attach the Grustrag Bolt.

It also exists as a alternative skin for the Dethcube Sentinel, which is available for purchase for.

Arrival
Triggering an attack from the Grustrag Three requires the player have a death-mark. This is acquired by supporting the Corpus in at least five Invasion missions. Once the death-mark is acquired, the player will receive a message from Vay Hek, stating: Similar to the Harvester, the chance The Three will appear on any given Grineer Mission is additively increased with each player in the party with a death-mark.
 * "You continue to assist our enemies. Your meddling will NOT be tolerated."

Before appearing, the Lotus will try to warn you (with her transmissions distorted), and eventually pleads you to abort the mission, with the Grustrag Three appearing shortly afterward. Each marine has a chance to drop Neurodes, a component/blueprint for the Brakk (though this is dependent on the marine) a rare mod (such as Natural Talent), or a rare Fusion Core upon death. If The Three are successfully defeated, the Tenno will be directed to extraction, ignoring the missions original objective.

Quotes
Below is a list of quotes from transmissions by the Grustrag Three. (Still a work in progress, any help would be appreciated.)

Vem Tabook Shik Tal ​Leekter
 * "(Player's name) is the scrawniest of them all."
 * "Is (Player's name) the best they have?"
 * "This one is mine!"
 * "Shiny little pretty things you Tenno are."
 * "Starting capture." -Stated when capturing the downed target.
 * "Unacceptable!" -Stated when one of the Three is killed.
 * "Boring!"
 * "Are we sure these are the targets? They seem so inconsequential."
 * "Pathetic."
 * "Do not waste my time Tenno!"
 * "You are beneath us."
 * "Is one of them wearing a scarf?"
 * "Too easy."-Stated when capturing the downed target.
 * "That one will be difficult to replace." -Stated when one of the Three is killed.
 * "I want to play with these ones."
 * (Player name) your (Warframe) looks so...slippery."
 * "You are going to suffer for that Tenno." -Stated when one of the Three is killed.

Defeat
If the Grustrag Three manage to kill the player, the session will automatically fail, taking the player back to the main menu. A new inbox message from the Lotus will then appear stating that The Grustrag Three have attached a Grustrag Bolt to the player's warframe, crippling its powers when attempting to fight them. As long as the bolt is attached, it will lower that frame's damage against Grineer by 50%. Attached to the same message will be a Bolt Release blueprint to release the Grustrag Bolt.

Tips

 * The G3's corpses can be Desecrated to improve your chances of getting Brakk components.
 * Stay in cover when fighting, because the G3 will attack you one at a time: Leekter, Vem Tabook, Shik Tal.
 * The G3 are immune to abilities that ragdoll regular opponents (such as Vauban's Vortex or Zephyr's Tornado). However, Vauban's Bastille, Rhino's Stomp, Excalibur's Radial Blind  and Hydroid's Tentacle Swarm are confirmed to affect them. (Need confirmation of other abilities that work and do not work.)
 * The G3 are affected by Nova 's Molecular Prime, which strongly slows them down.
 * The G3 are affected by Rhino's Stomp, which suspend them a good while to take them down.
 * The G3 are not immune to Oberon 's Reckoning, which lifts them up and slams them down, dealing considerable damage. If still alive, they will be down on the ground, which is a good opportunity to attack them.
 * The G3 can not detect Loki while invisible and will be distracted by his Decoy, so he can easily snipe them. Additionally, Loki 's Radial Disarm knocks down and damages the G3 as if they were Infested Chargers.
 * The G3 are vulnerable to knockdown effects such as Zephyr 's Dive Bomb, allowing players to stunlock them.
 * Hydroid 's Tentacle Swarm causes the G3 to be locked down against the ground in a constant state of ragdoll, making it easy to safely kill them.

Trivia

 * Sometimes, while the bolt is active, your shoulder clan tag shows the Grineer Flag instead.
 * They are the first death mark enemy that consist of multiple entities. The Stalker and The Harvester only consist of a single entity.
 * Upon spawning all players are contacted by the Lotus. However only one player will have his mark consumed.
 * When spawning, all the players will be taunted, but then one player will be taunted mentioning his name and Warframe. This player will have his mark consumed and must do five invasion missions for the Corpus to regain it.
 * At the time of their introduction, The Three were guaranteed to drop a mod, a Brakk component and Neurodes all at once upon defeat, potentially giving players all the Brakk components in a single encounter. This has since been patched.
 * The Grustrag Three appear to have some heavy duty jamming technology somewhere on their persons, preventing the Lotus from properly warning any targeted Tenno, and keeping her in the dark as much as possible from what is happening.
 * The Grustrag Three currently have the same drop table, differing only in Brakk components.
 * The Grustrag Three seems to have similar armor components to those of Lieutenant Lech Kril. Although, this armor similarity could possibly imply a category of those for Grineer Commandos, an even higher tier of Grineer troops than those of the Elite Lancers or the Napalms.
 * Although Leekter wields a Manticore in battle, his codex entry depicts him using a Brokk instead.
 * The Grustrag Three can spawn in Orokin Derelict
 * According to Shik Tal's Codex entry the Grustrag Three have apparently cost Ruk an entire troop transport of Grineer troops, which Ruk views as incredibly valuable to him, and has stated he'd kill the Three on the spot for their treason, if given the chance.

Bugs

 * The Grustrag 3 appearing in a Survival mission can prevent the 5-minute interval rewards from being given for the rest of the mission.