Among the angels it is the Archangel of Justice Tyrael who is humanity's greatest defender. Wielding his sword El'druin against the Burning Hells, the Prime Evils would have enslaved Sanctuary and all of humanity long ago if Tyrael had not intervened.
Diablo: Book of Cain
Tyrael is an character in Diablo II, Diablo II: Lord of Destruction, Diablo III and Diablo III: Reaper of Souls. Tyrael is one of the five members of the Angiris Council which rules the High Heavens.
Tyrael is the archangel of Justice, and as such he was the most rigid of the angels, firm in his adherence to laws, rules, and order. He held to a single, unbending duty: to secure victory for the High Heavens in the Eternal Conflict.
He was renowned for being calm and controlled and meticulous in his execution of combat technique. The archangel carried out his impassive judgment through the use of El'druin, the Sword of Justice.
El'druin is, by all accounts, a unique weapon that can cut through any substance or foe in existence. There is, however, an exception: Some believe that the blade's edge cannot pass through or harm any being of righteous intent.
Despite Tyrael's fame as a warrior with few peers, it is nonetheless stated that he was at all times fair and impartial, as justice itself must be.
The Courts of Justice, Tyrael's abode in the High Heavens, are said to be akin to a great auditorium wherein angels congregate. Here they air grievances, come to terms, and strive to reclaim lost harmony or equilibrium. It seems a fitting institution for an angel who values integrity and balance above all things and will choose the path of righteousness in any situation, even if it brings harm to those he loves.
During the aftermath of the Sin War, the Angiris Council voted to decide the fate of Sanctuary. Only Tyrael's vote prevented the purge of all inhabitants of Sanctuary.
A pact was determined between the Angiris Council and Mephisto that none of both factions would intervene in Sanctuary.
Milennia later, a civil war within the ranks of the Burning Hells resulted in the exile of Diablo, Baal and Mephisto. The lesser evils, Azmodan and Belial, sent them to Sanctuary. Tyrael enrolled the help of powerful humans to hunt down the Prime Evils and to imprison them. Tyrael and the Horadrim used the Soulstones -- shards of the Worldstone -- to imprison the soul of the Prime Evils, and buried them in distant locations.
During the Darknening of Tristram, Diablo managed to corrupt King Leoric to commit vile crimes. Prince Aidan (the hero) was ultimately possessed by Diablo (Diablo, 1996).
During Diablo II, Aidan (known as the Wanderer) traveled east to free Diablo's brothers: Baal and Mephisto. Tyrael tried to prevent Baal's escape from Tal-Rasha's Tomb, but Marius was tricked into removing the shattered soulstone from Tal-Rasha's chest.
Later, Tyrael sacrificed in order to address the taint of the Worldstone after Baal corrupted it. Tyrael threw the sword of justice, El'druin, at the Worldstone. Its explosion decimated Mount Arreat leaving a crater.
For twenty years, Tyrael's essence floated in the nether -- unable to coalesce himself back to his former self. Jacob of Staalbreak, the avatar of justice, wielded El'druin and with the help of Shanar the Wizard managed to bring justice upon men. It's unknown still how Tyrael managed to recover El'druin and to materialize his light-sound-like being.
His actions were pivotal to ensure the Prophecy of the End Days didn't come to pass. After confronting Archangel Imperius and criticize the Angiris Council's inaction to save Sanctuary and in turn all of the High Heavens, Tyrael decided to strip away his wings and his immortality as a lesson to his brethren that the laws shouldn't govern them into inaction. The welfare of creation is above any law.
Imperius exiled Tyrael toward Sanctuary, and he fell through the sky -- colliding against the Cathedral of Tristram. After the Diablo III hero rescued Deckard Cain, the way to King Leoric's throne is cleared to find the source of the falling star. It turned out to be a man. He holds no memories of his past or the reason of his arrival. All he knows is that he had come to warn men.
A search for the three pieces of this man's sword begins, and the Hero and Leah find out that Maghda and her cult are also searching for the pieces.
Later, in New Tristram, Maghda captured Deckard Cain and the three missing pieces of the sword. With Leah's sudden burst of power before the anguish of seeing her uncle Deckard harmed, Maghda decides to retreat.
Unable to force Deckard to coalesce the sword pieces, Maghda decides to kidnap the man that fell through the sky. Before his last breath, Cain uses the Horadrim teachnings and possibly the Horadric cube to unite the three pieces of the sword. Before him was El'druin. Deckard recognizes it as an angelic weapon. Cain died, and the hero and Leah keep moving on to rescue the kidnapped man.
After defeating the Butcher, the hero rescues the man from the cult, and hands him the sword. El'druin restores his memories and now he remembers he is Archangel Tyrael. Throughout their journey, Tyrael helps the hero, Leah and Adria the witch restore Zoltun Kulle in order to access the Black Soulstone. It is determined that it is a crucial instrument to capture the Lesser Evils.
With the defeat of Belial and Azmodan, Adria showed her true colors and betrayed Leah and Tyrael -- using the Black Soulstone and her magics to transform Leah into Diablo, the Prime Evil. Diablo opens a gateway into the High Heavens, where he brings utter chaos. Millions of angels perished, and Diablo climbed his way into the top of the Crystal Arch to defile and corrupt it.
Upon Diablo's defeat, Tyrael claims this to be the dawn of a new age for men and angels. The Crystal Arch chose Tyrael to embrace his new place as the Archangel of Wisdom, for justice had been met that day.
As time went by, Tyrael could see with mortal eyes what other angels couldn't. Corruption was flowing through the High Heavens. The Angiris Council's attitude seemed abnormal. Tyrael determined that the Black Soulstone was creating dissonance upon the High Heavens, thus, he decided to summon forth five heroes of Sanctuary and the new Horadrim to stage a stealth-mission to steal the Black Soulstone from the High Heavens (Read Diablo III: Storm of Light).