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. They are composed of Vem Tabook, Leekter and Shik Tal.

Lore
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.

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 Grustrag have attached a Grustrag Bolt to your warframe, crippling your powers when attempting to fight them. 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.





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.