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 members are Vem Tabook, Shik Tal, and Leekter.

Arrival
The Grustrag Three will contact the player after the player has compleated 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:
 * "You continue to assist our enemies. Your meddling will NOT be tolerated."

Before appearing, the Lotus will try to warn you, and eventually pleads you to abort the mission. After that, the Grustrag Three appear. After defeating them the player will have a chance to obtain the Brakk blueprint and/or a Brakk part, as well as a potentially rare mod (like Natural Talent).

The Gustrag Three share the same spawn chance as the Harvester does and can spawn on any Grineer tileset.

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 corrupting device called the 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.

Trivia

 * They are the first death mark enemy that consist of multiple entities. Stalker and Harvester only consist of a single entity.
 * Upon spawning all player are contacted by the Lotus. However only one player have his mark consumed (Tested, but needs confirmation).