The Grustrag Three

 The Grustrag Three (Abbreviated as the G3) 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= Each marine is accompanied by a Carabus Sentinel, which are equipped with Laser Rifles to provide additional firepower. They do not have a name in game, only a health/shields bar, similar to Loki's Decoy.
 * -|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 attaches the Grustrag Bolt.

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

In General
Carrying a strong Specter (with Latron Prime or Latron) can greatly assist players in combat, acting as a second player.

The Jat Kittag can and will ragdoll all three, leaving them easy targets for the rest of your squad.

Utilizing Warframe Abilities
Mag's Pull, unlike with the cases with most bosses/assasins, can ragdoll all three Grustrag members in the process.

Rhino's Rhino Stomp can put all three members in stasis, giving you time to heal or opening them up for a quick cleaving.

Nekros' Desecrate can grant a better chance of getting a Brakk component.

Arrival
Triggering an attack from the Grustrag Three requires the player to have a death-mark. This is acquired by supporting the Corpus in at least five Invasion missions. Death marks are acquired separately for each invasion mission which requires you to fight for the corpus on Grineer offensive invasions at least five times in a row. Once the death-mark is acquired, the player will receive a message from Vay Hek, stating: Similar to the Zanuka Hunter, 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 they appear, the Lotus will try to warn you (with her transmissions distorted), and eventually pleads you to abort the mission (ignoring the mission's original objective in the process). 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 defeat. If The Three are successfully defeated, the Tenno will be directed to extraction, though the Tenno can opt to finish the original objective if possible.

Defeat
If the Grustrag Three manage to defeat 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% as well as increasing the Stamina loss from wallruns by 50%. Attached to the same message will be a Bolt Release blueprint to release the Grustrag Bolt.

It is possible to destroy the Carabus Sentinels of the G3 to prevent the bolts from being applied, it will instead abruptly end the defeat with a death.

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), as well as Mind Control and Chaos abillities of Nyx. However, Rhino's Stomp, Excalibur's Radial Blind, Nekros' Soul Punch and Hydroid's Tentacle Swarm are confirmed to affect them. Vauban's Bastille also affects them, allowing for easier kills. (Need confirmation of other abilities that work and do not work.)
 * The G3 can be ragdolled and launched through the air by slamming the ground with the Jat Kittag, this also gives you the ability to fling them off into oblivion for an instant kill (It has yet to be confirmed whether or not you lose the items they drop or get them in the rewards when they die this way).
 * The G3 are ragdolled by every single Kestrel throw, which makes the fight laughably easy.
 * The G3 are affected by Nova's Molecular Prime, which slows them down considerably.
 * 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 are not immune to Ash's Blade Storm, during which shields can be regenerated.
 * The G3 can not detect Loki while invisible and will be distracted by his Decoy, so he can easily finish them off. 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. This can also be used to knock down Shik Tal, preventing him from using his Riot Shield to defend himself, which is especially useful if fighting them using melee weapons.
 * Nekros' Terrify works on the Grustrag Three. They won't shoot aimlessly while under the effects of Terrify.
 * Valkyr's Hysteria will allow you to solo them, it works better combined with Rage and a high rank Vitality.

Trivia

 * Sometimes, while the bolt is active, your shoulder clan tag shows the Grineer Flag instead. If the player has no clan tag equipped, an active bolt will peculiarly add one with the Grineer Flag.
 * They are the first death mark enemy that consist of multiple entities. The Stalker and The Zanuka Hunter 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.
 * Their body meshes share arm and leg geometry with Lieutenant Lech Kril. The jarring quality difference is a giveaway.
 * Although Leekter wields a Manticore in battle, his codex entry used to depict him using a Brokk instead. This has been fixed for Update 13.2.
 * According to Shik Tal's Codex entry the Grustrag Three have apparently cost Ruk an entire battalion 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.
 * Like Vor, Alad V and Sargas Ruk, Shik Tal is voiced by [DE]Skree.

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.
 * A similar bug can occur during interception missions if the broadcast reaches 100% while you are fighting them. The screen that allows you to continue the fight or end the mission no longer appears and Lotus assumes that you wish to continue to fight, making the mission impossible to complete.
 * Using the Grustrag Bolt Release to remove a Grustrag Bolt may allow players to use their weapons inside the Liset.
 * Being healed by a health restore or an ally's ability whilst simultaneously being downed may leave players in a constant loop of being downed, and also unable to be revived.