Prior to Sin War
Rathma was Trag'Oul's first pupil, and even the first being to discover his existence. Throughout the centuries that precurse the Sin War it is assumed Rathma is learning about all aspects of the Balance of life, a skill more commonly known as necromancy.
Trag'Oul came into existence when the world of Sanctuary was created. He is depicted as a constellation of stars in the form of a dragon and exists in a realm that only himself, Rathma, Mendeln, and Uldyssian have seen. He is very powerful and is the supposed guardian of Sanctuary, though, as pointed out by Mendeln, seems to sit in the shadows and is not a very active guardian.
During the Sin War, Trag'Oul selects Mendeln as his second student. During the battle outside Istani both Trag'Oul and Rathma lend their power to Mendeln so he would have the strength to sends hordes of morlu and the raised dead of both the Edyrem and Peace Warders to their final resting places.<ref>Scales of the Serpent, page 304. Richard A. Knaak, 2007.</ref> When Trag'Oul and Rathma realize the High Heavens have discovered Sanctuary the dragon uses his enormous power to shield the world from the eyes of the angels.
Inarius, in his fury, later sends Rathma to the void. Trag'oul fearing for Rathma's safety, teleports Mendeln straight into the void to find him. With Trag'Oul's instruction, Mendeln finds Rathma and they are both saved but the effort exhausts Trag'Oul and he can no longer shield Sanctuary from the eyes of the angels.
After the final battle between the Edyrem and the Cathedral has ended, and Uldyssian has cleansed Sanctuary of both demons and angels the Angiris Council convenes and concludes that Sanctuary will be preserved but the Nephalem with the exception of the first-borns (Rathma, Bul-Kathos, etc.) will all have their memories wiped and the council will restore the world to the state it was in before any of the humans discovered their abilities that they might "start anew". As the council agreed, they restored to the world how it had been years before, and all humans memories were wiped... excepting one. Trag'Oul spared Mendeln his memories so he could remember the sacrifice of his brother, Uldyssian, and so he might pass on his knowledge of the balance to any willing students.
We later discover Trag'Oul is furious at the fact that he has to hide his presence from the Angiris Council, even as the fate of his world is being decided.
It is made known in the Veiled Prophet that Trag'Oul is only one of many worldly guardians. Meaning not that there are multiple guardians to Sanctuary, but there are other worlds as well as other beings such as Trag'Oul that watch over their worlds. Each of these worlds is dictated by the balance and like Sanctuary they fear the Great Conflict coming to their worlds.
Because the war is coming to Sanctuary, and if you interfere with what the Balance demands, it and all existence may be forfeit.
They left him, then, all the others who stood guard as he did over their separate worlds, left him with the knowledge that it was his Sanctuary whose imminent fate might decide theirs. <ref>The Veiled Prophet, page 168. Richard A. Knaak, 2007.</ref>
- Trag'Oul has undergone many spelling revisions in Diablo lore, he was called Trang-Oul in Diablo II in the set, Trang-Oul's Avatar and revised as Trag'oul in the Sin War novels. Since the novels are canon as of Diablo III, and as they were released after Diablo II, we can assume that the great dragon's name is indeed Trag'Oul. * Trag'Oul is the only dragon referenced in Diablo lore (though it is not known whether the guardians of other worlds are also dragons).
↑ Scales of the Serpent, page 304. Richard A. Knaak, 2007. ↑ The Veiled Prophet, page 168. Richard A. Knaak, 2007.