Darth Thanaton, born as ‘Teneb Kel‘ ironically mirrors much of the origin story as the Sith Inquisitor you play as in SWTOR.

Born into slavery within the Empire, Teneb Kel sought apprenticeship under the Lord Calypho. He was sent to Korriban to train at the academy where he would eventually excell at both the force and assassination.

However it would not be for long as his master soon became wanted by the Dark Council for his heretical and unruly meditations on the force. Lord Calypho was taken to Dromund Kaas where he was arrested, bringing great dishonor to Teneb as a slave-turned apprentice to a disgraced Lord.
Turning his eyes to the battlefield against the Republic, he headed to the front lines where his skills in force use and assassination proved useful to show his talents within the Empire. It was at this time that Teneb Kel joined forces with his own monster companion, an Abysin named Maggot.

After even more successes on the frontlines, the Council recalled Teneb to Dromund Kaas where they forced him to nearly kill his old Master, Calypho in front of the Council to prove his strength. It would be after this duel that Teneb recieved his greatest mission, personally from the Sith Emperor Vitiate himself:

To capture and execute the Sith Emperor’s apprentice Exal Kressh.

Teneb and Maggot headed to the Lenico system where he tracked and dueled Exal Kressh. They pair fought viciously but here Exal Kressh would reign victorious and defeat Teneb, leaving him for death.
During his defeat, on death’s door, he slipped into a force vision that revealed Exal Kressh‘s history.
He saw through the force Vitiate’s master plan, the ‘Children of the Emperor‘. The ability for the Sith Emperor himself to puppet and control select force users across the galaxy to enact his will, harnessing their life force as his immortality and omnipotence.

Maggot was able to aid Teneb in his rescue and the pair then tracked Exal Kressh to Korriban, arriving in orbit of the Sith homeworld, they traveled to the tombs below the Valley of the Dark Lords.
It was here that Teneb Kel would duel Exal a second time, striking her down as she denounced Vitiate‘s plan. Teneb would turn and then strike down his companion Maggot in the tombs, fearing that he would spill the knowledge of the Children of the Emperor.

Returning to the Dark Council, Teneb Kel spoke openly and candidly about his victory and what he knew of the Emperor Vitiate‘s secrets. Feeling at the mercy of Teneb‘s knowledge, the Dark Council agreed to promote him immediately from Apprentice to Darth on the Council.

It was at this turning point in his life that he abandoned the name Teneb, rebranding himself simply as ‘Thanaton‘ infront of the Dark Council themselves. Thanaton then immediately took his seat, as the leader of the Sphere of Ancient Knowledge. It was over his time in this seat that despite his past history, became a staunch Sith Traditionalist as he grew in both political power and influence within the Empire.

Opinion:
While his own backstory was relegated to comics and not explained in SWTOR, it does lend similarity to the Sith Inquisitors journey.
Calypho and Zash – Both were born as slaves and adopted by a powerful Sith Lord that was seen as a “non traditionalist” and were eventually defeated for practicing such behaviours.
Maggot and Khem Val – Both companions were seen as ‘monster’ aliens in the eyes of the Empire yet loyally stuck side by side with their characters.
Zash and Xalek – Both respective apprentices outwardly speak of killing their master.
Thanaton and Inquisitor – On both of their journeys they rise quickly from apprentice to eventual Dark Council member.
Exal Kressh and Lord Kallig – Both Kressh and Kallig induce force appearances onto the characters during the lowest point of their character arcs, giving them the rise they need to overcome their biggest enemy in their respective arcs.
Tombs on Korriban – Maggots story ends and Khem Val’s begins in a tomb within the Valley of the Dark Lords.
Character Class – Thanaton depicts an assassin in the comics and a Sorcerer in game, both choosable combat classes of the Inquisitor origin story.

Sources:
Star Wars: The Old Republic – Blood of the Empire Comic Series
Star Wars: The Old Republic – Inquisitor Class Story