Mathael

— Selathiel, writing on Malthael
 * "Much uncertainty surrounds the archangel Malthael. Once the calm guiding hand of wisdom, he was lost to us the moment the Worldstone disappeared. Unable to fathom that catastrophic event, he abandoned Heaven. His departure created a colossal fracture within the council, one that, to this day, remains unrepaired."

Malthael was a former member and leader of the Angiris Council. Once the Archangel of Wisdom, he became the Angel of Death.

The Great Conflict
As with the rest of the Heavenly Host and its leaders, Malthael participated in the Great Conflict and played a crucial role in it. He was the first leader of the Angiris Council and through it, angelkind as a whole.

Wrath
In one battle against the forces of Hell in Pandemonium, Malthael operated in sync with the rest of the Council, as they fought demonic forces led by Diablo. Malthael easily deflected the demons' attacks and used his weapons to immobilize Diablo himself. However, Imperius' hotheadedness led to the Lord of Terror being brutally struck down rather than being taken prisoner as Tyrael and Auriel intended to which would have been beneficial to the war effort. Malthael remained silent the entire time.

The Sin War
"Whatever the choice in the end, it does not matter for me... I abstain." — Malthael abstains from judging Sanctuary

Not all minions of Heaven and Hell were content to be bound by the Great Conflict. The result was that under the leadership of the angel Inarius and demon Lilith, a number of rogues stole the Worldstone and created their own realm&mdash;Sanctuary. Malthael became consumed with discovering where the artifact had disappeared to, and as the Archangel of Wisdom, was tortured by his inability to find it.

The coupling of angel and demon would lead to the rise of the nephalem. Following Sanctuary's creation, while it was originally hidden from the High Heavens, Malthael's mood darkened. Deckard Cain would later speculate that the existence of the nephalem clouded Malthael's vision or opened his eyes to a new truth, or that there was a connection between the nephalem and Malthael's chalice that the angel became aware of. Towards the end of the Sin War, Heaven became aware of the existence of Sanctuary, and in the final battle, the forces of Heaven, Hell, and the Edyrem clashed against each other. Malthael watched on, and beheld the existence of humanity.

In the aftermath of the battle, the Angiris Council convened to discuss Sanctuary's fate and that of its inhabitants. Malthael abstained from voting, but the end result was that Sanctuary and its denizens would be allowed to exist, free to choose their own path.

Wisdom's End
"Wisdom shall be lost." — An excerpt from the Prophecy of the End of Days, speculated by Cain to relate to Malthael

Malthael disappeared after the destruction of the Worldstone. Just prior to this, he had apparently taken a greater interest in humanity. His departure broke the unity of the Angiris Council. Some speculated that he haunted the Halls of Pandemonium, seeking answers to the unknowable mysteries of life and death. Regardless, after Diablo's defeat in his siege of the High Heavens, a now mortal Tyrael took Malthael's place as the Aspect of Wisdom.

In truth, Malthael had seen these events. He had seen the actions of Zoltun Kulle, a human who had embraced evil. He had beheld the latent power of the nephalem hero. He had seen that hero defeat the Prime Evil, an act that in all his years of war in the Eternal Conflict, he had never been able to match. Faced with such power, and beholding humanity as an abomination, Malthael resolved to become the embodiment of Death. With the Lords of Hell defeated, he reasoned that humanity was the most powerful "demonic" force left, as per their ancient nephalem heritage. With humanity's eradication, Heaven would stand alone, victorious in the Great Conflict. He established the Pandemonium Fortress as his base of operations. Imperius' lieutenant, Balzael, fell in league with Malthael. He also came to be joined by Urzael, who shared Malthael's views on humanity. The angelic maidens that Urzael had brought with him also pledged themselves to Malthael's cause.

Reaper of Souls
"No-one can stop death..." &mdash;Malthael to Tyrael Not long after Tyrael took on his former aspect, Malthael re-emerged, now embodying the aspect of death. He confronted his erstwhile brother and a group of Horadrim, who were in the process of containing the Black Soulstone. Malthael effortlessly slew the Horadrim and devoured their souls in the process. While he was able to defend himself against Malthael's initial onslaught, Tyrael could not harm Malthael and was also incapacitated. Seeing that Tyrael's soul was free of demonic taint (unlike those of humans), he did not see Tyrael as his enemy and ultimately decided to spare his life. After casting Tyrael aside, Malthael then approached the stone with intent on bending its powers to his will, and bringing an end to the Great Conflict once and for all. Tyrael (powerless to stop Malthael), declared that the nephalem would stop him. Malthael paused his approach and turned to face his mortal brother; responding that no one could stop death, before shattering the restraints and claiming the Soulstone for himself.

Afterwards, Malthael led his Reapers against Westmarch. By the time the Nephalem re-emerged to confront him, his plan to eradicate humanity was near completion.

Both Tyrael and the Nephalem were baffled as to why Malthael had forcefully taken the Black Soulstone. As the Nephalem fought to save what was left of Westmarch, Tyrael deciphered a sliver of the Black Soulstone that had been broken off and left behind. Eventually, the Nephalem tracked Malthael's location to the Pandemonium Fortress while Tyrael finished deciphering the Soulstone sliver. Much to the horror of his foes, Malthael was fundamentally altering the Black Soulstone to consume and contain all demonic essence in Sanctuary. This meant that humanity faced annihilation as their demonic heritage would be torn from their souls by the stone and their bodies ripped apart. Malthael continued his work from the heart of the Pandemonium Fortress, continually feeding off the increasing souls of the dead and growing more powerful with each passing moment.

As the Nephalem fought their way through the Battlefields of Eternity and gained entrance to the Pandemonium Fortress, Malthael taunted the hero, claiming the eradiction of humanity had already begun. The Nephalem, now empowered and wielding the forces of Death itself, confronted Malthael at the heart of the fortress. Malthael proved to be an incredibly powerful adversary, displaying masterful prowess with his weapons as well as bombarding the Nephalem with waves of undeath. To bring the battle to an end, Malthael summoned the Black Soulstone into the fortress and shattered it, consuming the essence of Diablo. While this granted Malthael a massive boost in power, he was still ultimately defeated by the Nephalem. Malthael's defeat was shown to be a painful, agonizing one; coupled by a blinding explosion of energy that destroyed the roof of the fortress. As he fell to the floor below, the souls of all he had consumed erupted from his body, and his body slowly disintegrated. This included Diablo himself who, thanks to Malthael's actions, was now free.

In-game
Malthael is the main antagonist of Diablo III: Reaper of Souls. He serves as the Act Boss of Act V.

Tips and Other Additional Information
The battle with Malthael occurs on a relatively small circular platform. There are health wells to the west and east. The battle can be divided into three phases. The following abilities are ones he uses during all the phases:
 * Charge attack - a charge that causes knockback and a brief stun, but the damage is low if any.
 * Whirlwind - a whirling ring around Malthael, causing heavy damage to anyone inside. Malthael is fully mobile when using that power.
 * Death Shrouds - slowly moving clouds of mist on the floor. These cause Cold damage to anyone passing through, but can be ignored by the Talisman of Aranoch (of which there is a level 70 version). They will dissipate over time, but they are also spammable, especially in the last phase.

Phase 1
From time to time, Malthael flies up and land in the center of the platform. From there, he starts releasing a swarm of glowing blobs, each moving in a straight line toward the platform's edge. The blobs hit quite hard and stun briefly, but are slow and sparse enough to dodge easily, especially if you stand close to the platform's edge. Note that Malthael can be damaged while he stands in the center.

Phase 2
There is no clear transit between that stage and the first one. However, instead of the blobs, Malthael periodically summons a group of souls at the edge of the arena. They quickly spiral toward the center of the arena, from where they fly out toward the edges. A hit by a soul is highly damaging and stuns briefly. The edge of the platform stays clear of the souls' initial spiraling, while the final stage can be dodged easily enough. Malthael also summons Exorcists. The damage they cause can be significant, especially with their teleporting. However, they do drop Health Globes when killed. Note that if you keep close to the edge to avoid the souls, it is possible to get pinned down, so it's best to have a skill like Teleport ready.

Once you bring Malthael's health low enough (somewhere below 50%), all the adds disappear. You see a cutscene of Malthael shattering the Black Soulstone and absorbing it into his body. This starts the final stage.

Phase 3
In this stage, like in the first one, Malthael will from time to time fly into the air and land in the center of the platform. From there, he will release a group of fireballs flying in all directions except behind him. Get into that area as soon as possible, because immediately afterwards, two lines of projectiles (possibly the Soulstone's shards) sweep out around him, starting at the front and then going left and right. The sector behind him is the only place that remains clear, and the damage is severe. Otherwise, the main difference is the larger amount of Death Shrouds - Malthael can now spawn them two at a time.

Development
Malthael's presence in Reaper of Souls was designed/dictated by Josh Mosqueira. He wanted Malthael (the game's antagonist) to balance out against the game's new protagonist (the Crusader).

Personality and Traits
Malthael was a gaunt figure, adorned in black robes and armor. As the Archangel of Wisdom, he was once an intelligent and noble being who was said to cherish all life. However, he came to be regarded as melancholic, evasive and even frightening, and was an enigma even to his own kind. He rarely spoke, was slow to anger and could be ponderous and slow to act, but was revered by other angels for his insight. When he did speak, his voice is said to have enraptured other angels through its melody and the wisdom it conveyed. However, as his demeanor darkened, his voice became chilling and could provoke thoughts of foreboding and feelings of angst. Mendeln was one such example, Malthael's voice filling him with thoughts of permanent, empty death. Malthael saw humanity as an "abomination" that shouldn't exist.

As the Angel of Death, Malthael became even darker, callously murdering those who opposed him and seeing things in black and white. His wings, once bright and purple, had become white and wispy, seemingly skeletal in their appearance. The method of his transformation was different from other fallen angels such as Izual.

Abilities
Wisdom was once Malthael's aspect, thus, he was bound to see all things. He so derived this ability from Chalad'ar, the Chalice of Wisdom. While Malthael was slow to anger, he was still a peerless combatant. Wielding a pair of sickle-like shotels, he was able to deflect enemy attacks with only the slightest use of force.

As the Archangel of Wisdom, Malthael was one of the most powerful angels in existence. As the Angel of Death, Malthael's power only grew stronger. In this new aspect, Malthael demonstrated the ability to rip souls, and rot bodies by creating a dense fog with his wings. He was also extremely in-tune with souls, and was able to sense any demonic taint within them.

Trivia

 * The Angel of Death is a figure in Biblical and Talmudic myth, often otherwise regarded as the Grim Reaper.