Once the Aspect of Wisdom, Malthael abandoned heaven after the Worldstone's destruction in seach of meaning of life. In secret, he had resolved to end the Eternal Conflict by becoming a Reaper of Souls and eliminating everything affected by demonic corruption, including humanity.
Malthael discovered a whirling vortex of human souls within the heart of Pendemonium Fortress, where the great Worldstone once rested. He transformed this into a Soul Prison and harnessed the spirits' energy to become the Aspect of Death. With the Lords of Hell defeated, he reasoned that humanity was the most powerful "demonic" force left now, as per their ancient nephalem heritage. With humanity's eradication, Heaven would stand alone, victorious in the Great Conflict.
Malthael is an Assassin with innate high sustain, who can deal high health percentage damage to his enemies, thanks to his trait, Reaper's Mark; this makes him a potent "tank-buster", since Warriors will take increased damage due their larger healthpool. However, he can still deal respectable damage even to Heroes with lower health.
Overall, Malthael excels against comps featuring Double Warriors and/or other Heroes with high healthpool (such as Nazeebo and Azmodan). He can deal absurds amounts of damage under the right circumstances.
Extract the souls of nearby enemies afflicted by Reaper's Mark, dealing 100 (+4% per level) damage and healing Malthael for 36 (+4% per level) per target hit. Heroic targets heal Malthael for an additional 3% of the Hero's maximum Health.
Malthael's already high self-sustain through Soul Rip, combined with increased healing from Ana's Biotic Grenade (especially if she goes with the Biotic Build) can make him even harder to take down. Furthermore, his damage and sustain can become achieve insane amounts if he is under the effects Nano Boost and its upgrade, Nano Infusion, when using Tormented Souls.
Having an ally who can make it easier to get close to enemies and apply Reaper’s Mark is a huge benefit for Malthael, and few do this better than Arthas. In addition to powerful slow and root effects that keep enemies from escaping, Arthas’ ability to self-heal and stay in the fight is a great match for Malthael.
Malthael is a great candidate for Bestow Hope, as he can easily apply his Reaper's Mark to many enemies at once, which in turn generate constant energy for Auriel. He can be more effective than Lunara in this regard, specially when fighting against opponents with high health, such as Warriors.
With a strong initiation, effective crowd control and great peeling potential, E.T.C.'s team fight presence opens various windows of opportunity for Malthael to pick off opponents out of position.
Johanna is very effective at peeling and crowd control, which is great to protect Malthael while allowing him to spread his Reaper's Mark. Condemn can effectively pull enemies together so Malthael can hit as many as possible with Tormented Souls.
Lunara's Nature's Toxin combined with Reaper's Mark is perfect to slowly execute Heroes as they try to escape. Since damage-over-time effects stack, this results in an outstanding sustained damage that slowly cripples opponents.
Malthael's already strong self-sustain makes him better paired with allies that can mitigate or prevent damage rather raw healing. Tassadar offers great utility, with Force Wall being both useful to trap victims of Reaper's Mark, as well prevent divers from attacking Malthael.
The bonus Armor and Invulnerability effects provided by Uther can serve to amplify the value of Malthael’s self-healing, allowing him to stay in the fight and continue draining the life from his enemies. Uther’s powerful Stun and Cleanse effects also have great synergy with Malthael: keeping him free to move around, making enemies easier to reach, and making it harder for them to burst Malthael down.
Malthael is very difficult to kill when enemies aren’t able to focus him, and Varian’s Taunt heroic provides a reliable way to ensure that they can’t. The Armor and amplified Healing bonuses Varian’s Banner can provide also have tremendous synergy with Malthael’s powerful self-healing.
Having a unusual high health for a non-Warrior, Azmodan's limited movement and lack of crowd control allows Malthael to stick on him. Additionally, Azmodan's ability to summon minions means Malthael can get extra health from Soul Rip.
Boasting from the highest healthpool in the game, Cho'gall takes severe damage from Reaper's Mark. In addition, their large size makes them an easy target for Death Shroud.
Rexxar is an easy target because of Misha. Since she counts as a Hero too, Malthael can easily exploit her AI and feed on her, plus getting the bonus heal from Soul Rip; he can also quickly farm Misha to complete his quest talents.
Samuro's clones are also easy targets to Soul Rip, which allows Malthael to exploit their AI and feed on them. Similarly to Misha, they can also be used as farm material for Quest Talents.
In a similar fashion to Azmodan, Xul's ability to summon minions results in Malthael having extra sources of heal through Soul Rip. Since Malthael's damage relies more on his trait than basic attacks, he can stay from a safe distance, avoiding Xul's gap closers (Cursed Strikes and Bone Prison).
Malthael truly shines when fighting opponents with high health, namely Warriors. Since Zarya has poor movement speed and lacks any form of crowd control outside of her Heroic Abilities, she is an easy target for Reaper's Mark.
Stuns and Knockbacks make it much harder for Malthael to apply Reaper’s Mark to enemies and heal himself with Soul Rip, and Auriel brings both with her Detainment Strike ability. She can also use her Crystal Aegis heroic to counter Malthael’s Last Rites.
In a similar fashion to Lunara, Falstad can effectively kite Malthael and outdamage him in a duel. Mighty Gust can easily put Malthael out of position and prevent him from spreading Reaper's Mark.
Heroes with powerful burst damage can blow up Malthael before he has the chance to get much healing from Soul Rip. Jaina takes this a step further by also slowing Malthael from range, making it difficult for him to get close enough to apply Reaper’s Mark.
Despite his outstanding self-sustain, Lunara can dish out damage faster than Malthael can heal. Her natural increased speed allows her to easily avoid and kite Maltahel, preventing him to inflict her with Reaper's Mark, thus negating his only form of gap closer, Wraith Strike.
Ranged auto-attackers can deal consistent damage to Malthael while staying far enough away to avoid Reaper’s Mark. As such, Tracer is exceptionally difficult to pin down, making it tough for Malthael to have much effect against her.
Valla's aggressive playstyle and high burst can cripple Malthael before he has any time to react, capitalizing on his poor movement options and lack of crowd control abilities. Should Valla pick Rain of Vengeance, it will make her even deadlier.