With the information provided by the Chronicle, I cannot help but to wonder if the Aspect of Death, Deathwing, was uniquely susceptible to the warlocks’ fel magic. Fel magic is the magic that all titans (living worlds) were uniquely susceptible to and one titan known as Azeroth is where the Aspect of Death inhabited.
Aspect of Earth
Deathwing was once the proto-dragon Neltharion and he helped his fellow proto-dragons — Alexstrasza, Malygos, Nozdormu, and Ysera — defeat an abomination known as Galakrond. Now, after the proto-dragons defeated Galakrond, the titanic keepers — creatures created by the living worlds — journeyed to the frozen tundra where the battle had taken place. Acting as conduits of their creators’ powers, the gathered keepers bestowed the blessings of the titan Pantheon upon each proto-dragon. The Keeper Archaedas in particular asked his creator, Khaz’goroth, to bestow some of his vast power upon the indomitable proto-dragon Neltharion. Known afterward as the Earth-Warder, Neltharion was given charge over the mountains and deep caverns of the earth. He embodied the strength of the world — a world that was uniquely susceptible to fel magic as a titan.
I presume that fel magic corrupted Neltharion and ultimately destroyed him. Prior to his world even existing, there was a void. It is this very void that is partially (or ultimately) responsible for the existence of demons and worlds devoid of order. When regarding to World of Warcraft, the titan Azeroth was invaded by demons and Neltharion convinced his fellow Aspects to sacrifice a portion of their power and infuse it into the Dragon Soul, a weapon of his own design that he claimed would focus their powers and scour the demonic Legion from the face of Azeroth. Unbeknownst to the other Aspects, Neltharion had fallen prey to whispers of the Old Gods. Over the ages, the vile entities’ influence had tainted the bedrock surrounding their underground prisons. Neltharion’s innate ties to the earth made him uniquely susceptible to this evil.
Darkness had gradually pervaded the Aspect’s once-great heart. His descent into torment and madness led him to create the Dragon Soul, although it would later be called by another, more fitting, name: the Demon Soul. Amid a furious battle between the Legion and the night elf resistance, the five dragonflights made their final assault against the Legion. Working in unison with his comrades, Neltharion unleashed the full might of the empowered Dragon Soul and utterly decimated the Legion. Yet just as hope began to swell among the defenders, Neltharion turned the weapon against his own allies. Eventually, Neltharion would be known as Deathwing. His treachery would curse his own black dragonflight to a life of fear and seclusion. In the coming ages, they would find themselves hunted to the brink of extinction by the other flights due to Deathwing’s unforgivable betrayal.
Nekros, Thrall, And The Death Of Death’s Aspect
Eventually, a demon lord known as Kil’jaeden promised an orc known as Gul’dan godhood, teaching him the arts of a warlock, and helped him form the necrolytes. I presume that the goal of these necrolytes is the same as the goal of the very warlock who formed them as a group, Gul’dan. Anyways, one of the orcs who took up the ways of Gul’dan is known as Nekros and he eventually had learned much about the Demon Soul artifact from dreams and visions sent by Deathwing. The black Dragon Aspect had taught the orc of the Demon Soul’s true power, and he had instructed him in how to wield it. The greatest knowledge Deathwing had shared with Nekros was that the relic could be used to enslave Alexstrasza and other Dragon Aspects.
Nekros enslaved Alexstrasza but, with the interference of the mage Rhonin, Alexstrasza was eventually freed and was able to kill Nekros. After some timely wimey nonsense shenanigans, the Demon Soul of some alternate timeline was taken to the main timeline and used by the shaman Thrall to destroy the Aspect of Death. Whether this version of the Demon Soul had fel energies like the Demon Soul of the main timeline is yet to be revealed, but I wouldn’t be surprised if fel energies ultimately corrupted and destroyed the former aspect of a world that was uniquely susceptible to fel magic itself.