Plague of corrupted blood

The plague of corrupted blood or great plague was a disease that broke out in the Zandalari Empire during the final era of the Council of Tribes, after a foolish troll tried to summon Hakkar the Soulflayer. The plague tore through the empire, killed thousands of trolls, and began one of the most terrifying periods in Zandalar's history.

It became too risky for mortals to handle the dead, and golems were summoned for the grim work of preservation and burial. It was also deemed that the bodies of the plague victims, too polluted for proper burial, would instead be burned and kept in special urns. It is believed that the Soulflayer's touch may still linger within Kings' Rest where the urns are kept, thus a placard within the crypt tells anyone who may become infected to leap off the bridge to their death.

After the great plague, there were still those who clung to the magics of the blood god, having taken the "Path of Blood". They tested their magics and perfected them. One such relic was a Sanguinating Totem capable of draining the life and blood from enemies and infusing the caster with it, altering them in unsual ways. This was the dawn of who the Zandalari call, "the blood trolls".

The constructs within Uldir studied the plague in search of a cure, but found that only isolation and extermination was effective. When Uldir's safeguards cracked apart, the blood plague was accidentally combined with a sample of G'huun's blood, creating Vectis, an entity that hungered for fresh victims.

Speculation
This plague may be the same incident mentioned in Paragons of Power: The Confessor's Bindings, where shortly after the fall of the Gurubashi Empire around 1,500 years ago, the Zandalari almost succumbed to an "evil within their own tribe", which led to them "banishing the trolls who would destroy their own kind". This evil might have been Hakkar summoned to Zandalar, and the banished trolls would be the ones who summoned him. However, Hakkar was summoned to Zandalar "many millennia ago", which would be an odd choice of words if it was the same incident as this one which occurred 1,500 years ago.