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=
 * -|Shik Tal=
 * -|Vem Tabook=

Arrival
The Grustrag Three will contact the player after the player has completed at least five Invasion missions supporting the Corpus, which will trigger a death mark, much like Alad V for the Harvester. The player is contacted by Councilor Vay Hek, stating that the player's meddling will not be tolerated, reading: 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 mission can then be completed normally.

Defeat
If the Grustrag Three manage to kill you, the mission that you are on will automatically fail, taking you back to the main menu. You will find a new inbox message from the Lotus saying that The Three have attached a Grustrag Bolt to your warframe, crippling your powers when attempting to fight them. As long as the bolt is attached it will lower that frames damage against Grineer by 50%. Attached to the same message will be a Bolt Release blueprint to release the Grustrag Bolt Below are the manufacturing requirements needed to build the Bolt Release.

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 some crowd control abilities like Vauban's Vortex. However, Vauban's Bastille is 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.

Trivia

 * 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 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.