“You're not supposed to be in here! You're going to ruin the surprise!”The following section contains spoilers.
“Orokin secrets cannot remain secret forever! Start talking!”The following article/section is conjecture. Content is subjected to change/removal as the game progresses. Please do not use this article for critical in-game information.
Dr. Tengus is a renowned Grineer scientist, known to the Tenno as the Director of the Grustrag Reconditioning Facility and for callous innovation and remarkable creativity on the subject of genetics and psychology.
The Grustrag Three were formerly regular Grineer marines who, while aboard a troop transport, suddenly slaughtered their comrades, claiming that they were growing "impatient for the killing". After being committed to the Grustrag Reconditioning Facility, Facility Director Dr. 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, General 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.
Dr. Tengus uncovered the Infested and studied them for use as possible bioweapons. Marines were used to test the specimen's combat efficiency, and it was noted that the Marines that used fire-based weaponry survived the longest.
When the Tenno investigate the so-called bioweapon, Tengus deliberately releases the Infested as a bid to destroy the Tenno. The Tenno use a Cascade Bomb to eliminate the Infested outbreak, but the Lotus warns the Infested threat has only begun.
Meanwhile, Tengus notifies the Grineer Queens and falsely claims the Tenno were responsible for the Infested outbreak and had acquired spores to use against them.
In an attempt gain favor with the Twin Queens and prove his own worth, the up-and-coming young Councilor Vay Hek collaborated with Dr. Tengus to create the Grineer Ghouls - hastily-bred clones intended to be seeded from the orbit onto a targeted world, which they would proceed to conquer without the need for reinforcements or support.
In an act of betrayal, or perhaps as a deliberate stunt to showcase their ferocity to the Queens, the treacherous Tengus kicked Vay Hek into the Ghouls' enclosure, to be torn apart by the cannibalistic creatures. While the Queens were impressed by the display, they ordered Tengus to reconstruct Hek with cybernetics, turning him into the creature the Tenno eventually encounter. It is likely Dr. Tengus continued working under Vay Hek after this point, as there are several mentions of the former sending him subjects (Tenno or Ghoul Defectors) for dissection.