Abyssal Plague

The Abyssal Plague is a multiverse-spanning series of seven novels and one novella, published by Wizards of the Coast between 2010 and 2012. The event revolves around the Voidharrow, an entity from a dead universe that is unleashed across multiple worlds. Two of the novels in the Abyssal Plague series are set in the Forgotten Realms.

"In a dark dimension beyond the known planes of existence, the Chained God thirsts for freedom. Infusing his will into the residue of a long-dead universe—scarlet liquid shot through with veins of silver and flecks of gold—he sends this liquid crystal between worlds to prepare his way. But his will is not the only force that drives the Voidharrow.

Imbued with the power of the Voidharrow, the dragon Vestapalk creates a horde of demonic minions to spread the Abyssal Plague across the Nentir Vale. But the threat is not contained within a single world: Faerûn and Athas must contend with their own outbreaks of tainted demons and virulent plague. Will the Voidharrow consume the multiverse? Will the Chained God break free? Or will the heroes of the age put a stop to the Abyssal Plague?"

Description
The Abyssal Plague event was divided into three parts:
 * Origins: the beginning of the Plague, and introductions to most of the main characters
 * Plague Strikes: the main trilogy of novels, set on the world of Nerath
 * Plague Spreads: how the Plague affected the worlds of Toril and Athas

Origins
The Chained God Tharizdun stirs in his prison in Pandemonium. On the world of Nerath, Tharizdun makes contact with one of his disciples, who is instructed to use a fragment of an artifact known as the Living Gate to pierce Tharizdun's prison and allow the god to free himself. As Tharizdun's plan is underway, the benevolent deities Ioun and Pelor move to foil him through their own mortal champions, among which is Demascus, the Sword of the Gods. The champions chase the disciple and his minions from Nerath unto Sigil and finally into Pandemonium itself, but are unable to stop the ritual in time. The entity known as the Voidharrow slips out of Tharizdun's prison, imbued with a fraction of the god's willpower. The Voidharrow manifests itself as a liquid, crystalline substance, that corrupts and twists those who touch it with Abyssal Plague, and turning them into plague demons. The champions of Ioun and Pelor catch up to the disciple and his now-twisted minions, and in the ensuing fight, parts of the Voidharrow fall into portals leading to the worlds of Athas and Toril. Demascus also falls through one of the portals.

On Nerath, the Voidharrow is captured by a wizard. Through a convoluted series of events involving the green dragon Vestapalk, Orcus, and an agent of the Voidharrow known as Nu Alin, the Voidharrow is eventually freed and possesses Vestapalk.

Publication history
Follow up with Sword of the Gods by reading Spinner of Lies. (Part of the Rise of the Underdark event)

Before reading Shadowbane, consider checking out Downshadow (novel) (Part of the Waterdeep (series)) and then after Shadowbane, follow up by reading Eye of Justice.

The Ghost King, Whisper of Venom, and others stand as prequels to this storyline.