Born For Greatness
Before Thel joined the Covenant army, he was born into a very powerful family. The Vadam family are the rulers of the powerful State of Vadam on the Sangheili home-planet Sanghelios. While living with his family, Thel early on learned about ruling, as well as taking and giving orders.
Like so many of his Sangheili brethren, Thel joined the Covenant military at a young age and became known as Thel 'Vadamee, the –ee suffix is an honorific indicating that the Sangheili has enlisted in the Covenant military. Early in his military career, he received a leadership role, while simultaneous being promoted to Kaidon for the Vadam Keep. Early in his rule, he was attacked by three Sangheili assassins wielding Energy Swords. He managed to kill all three without even being scratched, but bluffed that he left one alive in order to determine who ordered the attack. He then killed the Vadam elder for ordering the failed assassination, and ordered all of his line to leave the keep by sunrise or they would otherwise face execution.
Ordered into battle, Thel was given command of the Retribution's Thunder to fight in the Battle of Charybdis IX whilst being overseen by the Prophet of Regret. After the UNSC forces gave up the assault and instigated the Cole Protocol, but not before Thel found out that the UNSC were using modified Covenant weaponry. He was ordered by the Prophet to investigate this, and was given the location of a notorious Kig-Yar smuggling den. Here he apprehended the Kig-Yar leader Reth who told them that his smuggling was sanctioned by the High Prophets as a ruse to find their homeworld.
After many victories against the enemy, Thel was eventually granted the mantle of Supreme Commander. In this capacity, Thel flourished, conquering and laying waste to numerous human colonies in the name of the Covenant. During his time as a Supreme Commander, Thel was responsible for over 1 billion human casualties, the loss of at least 7 human worlds, the destruction of over 123 human vessels during fleet action and the deaths of over 23,000 UNSC personnel.
Being widely considered to be the Covenant's greatest warrior, he was sent to destroy Reach with his fleet. While eliminating any opposition from the human stronghold, he noticed one human vessel which was attempting to flee the battle; Thel ordered all ships within his command to follow. This ship led them all to a gigantic ringshaped world which was quickly identified as being Forerunner. Still in pursuit of the human vessel, Thel's forces landed on the ring and began searching for the vessel.
While doing his search however, some of the forces under his command accidentally opened a vault which held Flood spores and thus releasing a force none could understand. Commanding the Seeker of Truth, he had his ships begin evacuation of the ring upon the release of the Flood, but the parasite managed to capture a ship and infect a San'Shyuum, the Minister of Etiology. The sudden widespread evacuation led to confusion as ships became infested, and he was unable to prevent the destruction of the ring and was forced to abandon it. The abandonment saved his life, but also resulted in the capture of the newly arrived flagship, the Ascendant Justice, by John-117, and he was called before the High Council to answer for these failures.
Thel was stripped of his rank, title, name, honour, and branded a heretic for his failure in safeguarding Halo. The Mark of Shame was branded upon his chest in front of many of the Covenant, and he was sentenced to death. He was sentenced to be hung by his entrails and his corpse paraded through the city as an example to others. However, when he was brought before the High Prophets of Truth and Mercy, they intervened and offered to commute the sentence if he would become the new Arbiter.
On the Battlefield Once More
Thel's first combat assignment as the Arbiter was to stop the Heretic uprising based in a Forerunner gas mine in the atmosphere of Threshold. They had expected him to die in the mission so this was part of his sentence. Against expectations however, he succeeded in defeating the Heretic leader Sesa 'Refumee by forcing him in the open by putting the gas mine in free-fall to the planet. Sesa told several times during the final battle that he had a good source for his "heretical ideas", which turned out to be the Monitor 343 Guilty Spark, a holy Oracle in the Covenant religion. With the uprising suppressed, the Jiralhanae Chieftain Tartarus extracted Thel and used his Fist of Rukt to capture the Oracle. His actions, which involved throwing the Oracle into the Phantom recklessly, angered Thel enough for him to remind the Chieftain who he was dealing with. With Tartarus, he left the gas mine via Phantom, as High Charity and the fleet were leaving Threshold's system.
After his surprising return to High Charity, Thel was sent to a newly discovered sacred ring to procure the Sacred Icon. Before the mission, Thel witnessed the replacement of the Sangheili Honor Guards with Jiralhanae Honor Guards. While on his mission, Thel encountered several human forces and the remains from a battle between UNSC Marines, Sentinels and Flood Combat Forms, and witnessed Miranda Keyes and Avery Johnson retrieving the Index. After the initial skirmish with Miranda and Avery, Tartarus captured Miranda and claimed the Index for himself. Tartarus informed a wounded Thel about the Jiralhanae's intended genocide of the Sangheili, by orders of the High Prophets… Tartarus then cast Thel down a large shaft, intending to kill him. Instead, Thel was caught in mid-air by the Gravemind who wanted to speak to both him and John-117, and put them on the path to keep the High Prophets from destroying all sentient life by activating the Ring.
Cancelling the Great Journey
At first Thel did not believe the tale, that the "Great Journey" did not exist, and in fact, was a synonym to destruction on a massive scale. Discovering the consequences of the San'Shyuum' betrayal of the Sangheili and the slaughtering of his species by the Jiralhanae, the Arbiter joined forces with several Sangheili, as well as a small number of Unggoy, to raze a Jiralhanae encampment. During this fighting, Thel made a shaky truce with Avery Johnson in order to prevent Tartarus from activating the ring. Together, they managed in just this, and came across 343 Guilty Spark who told everyone what the ring's true purpose was. With this revelation, Thel tried to convince Tartarus to turn against the San'Shyuum. Tartarus, however, blinded by his fanatical belief in the Great Journey, as well as his hatred for the Sangheili, ultimately chose to side with the San'Shyuum. In the battle that followed, the Installation was activated and threatened to destroy all sentient life in the galaxy. Thel, with the aid of Avery and several Sangheili, slew Tartarus, allowing Miranda to stop the activation in time. The shutdown, however, activated a failsafe system; the rest of the Halos were set to standby mode, and could only be activated remotely from the Ark. Thel then escaped the Flood-infested installation with the surviving humans and Sangheili forces.
After Tartarus' defeat on Delta Halo, Thel officially changed his surname to 'Vadam, symbolizing his resignation from the Covenant; the rest of the Sangheili followed suit.
Upon arriving on Earth with Avery and several allies, they started to search for John-117, who had crash-landed somewhere in the jungle nearby. Together with John and a small UNSC force, Thel helped clear the UNSC base Crow's Nest from Jiralhanae forces and would join John in the Battle of Tsavo. Having learned the truth about the Covenant's excavation, and having fought of a considerable Flood force, Thel joined John in transporting to the Ark and finish the Prophet of Truth there. The Prophet of Truth would later be killed by impalement on Thel's very own Energy Sword, proclaiming that the Covenant would not survive. Thel would later return to Earth, having earned the respect of Lord Hood and then set his sights to home.
Swords of Sanghelios
Having left Earth behind him, Thel planned to return to Sanghelios and restore order, as well as campaigning for an indefinite peace with humanity. His main opponents was the Servants of the Abiding Truth, an ancient order which followed the pre-Covenant belief that Forerunner relics were sacred and not to be touched. When Thel allowed a Special Ops team from ONI to deploy on Sanghelios in search of a human researcher, many Sangheili opposed him. This act, which was sacrilege in their eyes, gave them reason to join the Servant's. This resulted in Thel's forces being severely outnumbered, and the end seemed neigh. In the final moments both sides were bewildered as a gargantuan human warship, the UNSC Infinity, made its debut and entered into position around Sanghelios. Admiral Hood personally offered to assist Thel in the defence of his keep. While he wished to refuse such charity, he knew it would mean defeat, and thus he accepted. The Infinity would then destroy all opposing forces, and restored Thel as the leader.
The Sangheili-Jiralhanae war was still raging on, with no clear end in sight. However, Lord Hood proposed a peace conference in which the Jiralhanae leader Lydus and Sangheili leader Thel would talk, mediated by Lord Hood himself and a small number of SPARTAN-IV fireteams. During this conference however, Jul 'Mdama's Covenant attacked. Surprised by the ambush, Lydus believed that he was deceived by Thel, but Thel quickly reassured him that the Covenant attacking them are not his own troops by saving Lydus' life from the sights of a Kig-Yar sniper. The delegation, fleeing from the assault was seemingly cornered in a plasma refinement facility. Without a communication link with the Infinity, the SPARTAN-IVs fought against the Covenant and procured a Lich which they used to fly away. Later Thel, Lydus and Hood parted ways, not having reached an agreement.
ONI SPARTAN-IV Jameson Locke would later reach out to Thel who was looking for John-117.