The Unlife is the power of true evil. It seeks to destroy and create chaos to feed upon. It does not wish to simply kill, but to devour the spirit of the dying. There is a fate no worse than having your soul utterly destroyed.
Why do beings serve the Unlife? There could be many reasons: greed, impatience, the desire for vengeance, the lust for power. All of them also suffer from the arrogant presumption that they will be able to drink from the well of the Unlife and yet retain their freedom of mind and spirit. Almost all are wrong. Only the most powerful of dragons were thought to be able to use its power with impunity. Even the Lords of Essaence were known to fall under its sway eventually.
The House of the Rising Phoenix was formed to combat the Unlife. It remains to them and others as a concept of evil. While the Unlife is not directly supported any longer, the society called the Council of Light and its soul draining abilities are based on the Unlife. The roleplay and power source of that society remain part of the dark underworld of the game's structure.
The undead, while not the Unlife itself, are most often created through the power of the Unlife in manifestations of souls placed or trapped in host bodies. Outside of this, Luukos, to which some accredit the sphere of Undeath, and numerous Lornon Arkati, such as the potential Ur-Daemon Marlu, are known to use powers in line with the Unlife. The source of current in-game lore for this power is the Ur-Daemon. While none have manifested in the game, some are of the opinion that demons and planar enemies, such as Vathor, Ithzir, the Vvrael, and others, represent lesser manifestations of this power.
For those who oppose the Unlife today, they seek to fight evil in its various forms, be it demonic, invading armies, the occult, the immoral, or those who seek to oppress free will. This is a short list as presented, and while not all forms of evil draw from the Unlife, it's safe to assume the forces of good will combat the various manifestations of darkness.
The Unlife exists, if not in name, then in spirit.
Behind the Scenes
The Unlife was an I.C.E. Age concept of GemStone III that was removed in 1995. In the Shadow World historical setting, there was a cosmology of "planes of existence" along an axis of Order and Chaos. The Lords of Orhan (Liabo) originated in a higher plane of Existence, with the essence of the reality leaking through and allowing magic to exist near this world. Material planes of existence were on the lower end of the ordered side, and the more chaotic planes were the home of demons and the dark gods. These deities were extra-planar in origin in that timeline, as perhaps even all of them ultimately were.
The furthest extreme of this more chaotic essence was pure Anti-Essaence, which was inherently corruptive and destructive of all the rest of existence. This "heart of Chaos" is metaphorically known as "The Unlife," where exposure to the darker and more chaotic essence is inherently corrupting, warping the mind and body of those exposed like a kind of metaphysical radiation poisoning. Corrupted or "tainted" souls would be cursed with Undeath, for example, with an inherent need to destroy or devour the living. This is why the Order of Voln was originally about cleansing, and likewise for the Council of Light.
The Shadow World source books are non-committal about whether or not the Unlife has any self-awareness or will. Generally speaking, the phrase is used to refer to things from the dark end of existence rather than "The Unlife" itself or organizations with corrupted figures who want to destroy civilization or the world entirely. Sometimes the Dark Gods of Charon (Lornon), Dragonlords, and warlords, are spoken of as Unlife and other times as something distinct from it. Most of the time it is merely a metonym or synecdoche for evil cults, such as The Iron Wind, with the specific goal of utter annihilation. As a concrete example, these cults are said to understand its "true purpose", while members of the Jerak Ahrenreth cabal, who use dark magic, would instead mostly call these cultists madmen, yet the crystal shards in their fortresses they use to subvert the Eyes of Utha for more dark power is helping allow in more of "the Unlife," and, ultimately, they are unwittingly assisting the destruction of the world. While the "Unlife" as an explicit agenda is not part of the modern lore, its fundamentals are still in the game. Concepts of taint, anti-mana, and extra-planar bleed through, as well as the violence of holy and unholy power.