Sahjook is a Sangheili Ultra, later promoted to Zealot, under command of Rahgath. He volunteered to fight alongside Sev and later shows his loyalty to him by leaving in a Spirit after the battle concludes. He never speaks a word, which makes him more menacing.
He makes his first debut in Act 4, where he is spoken to by Firelight about his mysterious appearance and seems aggravated with Rahgath's sudden affinity towards him. However, when he does not respond, Firelight claims he does not like secrets in his army and orders Sahjook to speak to him. However, he continues looking forward. Sev then appears and tells Firelight that if he has a problem, he must speak to the both of them. They then look at Firelight who goes on about Sahjook's possible promotion and claims he is to be the only Zealot. He then goes off to say Rahgath would do no such thing. As Firelight turns away, Rahgath enters. Firelight backs up, surprised, allows Rahgath to pass. Rahgath then tells Sahjook to lead yet another assault on the island. Sahjook then walks away. Later on in Act 4, Sahjook is seen on the island, where he and a team of Majors and Minors, assault the island with the intention of killing the SPARTAN's. After a brief firefight, Mark fires two rockets at Sahjook, in which he simply deflects by firing his concussion rifle. He is then pursued by Swift, who is then distracted by two Minors. One of them then kills Mark from behind. Swift, who is enraged by this, turns rampant and then starts to massacre most of the Sangheili. He then tries to kill Sahjook, but is knocked by his gravity hammer. Swift then flees the island along with the others.
Later in Act 4, Sahjook is promoted to the rank of Zealot to replace Firelight. Sahjook leads a strike force of Sangheili to reclaim the spire. In the battle, all of the rogue Sangheili are killed and Sahjook then hits Thel off of the Spire, killing him as well. Later, Sev and Sahjook are sent to clear out the human outposts in the highlands.
Sahjook later took place in a battle between Whisper, Brass, Al and Memphis; fighting with Sev and a force of Sangheili. Sahjook fought Al and Memphis, pushing them both back single-handedly.
In Act 5, during the final battle between the human and Sangheili forces, Sahjook sneaks behind Perseus, stabbing him in the back. Nathan, however, shoots him in the chest with a sniper rifle, causing him to retreat momentarily. Sev arrives and kills Donelly and battles Nathan, before Sahjook assassinates him.
Later in Act 5, Sahjook and Sev lead a small troop of Special Operations Sangheili into a large Forerunner room. Sahjook and Sev battle Swift and Nightflash inside. While Sev and Swift battle, Sahjook takes on Nightflash. After a brief battle, Nightflash briefly knocks Sahjook down, and as he stands up, he notices that Swift is incapacitated. As he approaches Swift, ready to finish him off, Nightflash throws his energy sword, impaling Sahjook in the back and killing him.
Sahjook appears to be much more skilled than other Sangheili with the rank of Ultra, such as Thel and Ukran, as he kills many humans at the island and is only eventually killed by Nightflash when he is distracted.
He is also much more intelligent in battle than practically any other Sangheili that has been seen in combat as he utilises his weapons in an effective way, as shown in Act 4 when he uses his concussion rifle to deflect two rockets and also in Act 5 when he deflects an incoming Banshee away from him.