Uldir came out with the Battle for Azeroth expansion and was the first raid. The Halls of Control are located in Nazmir at the Heart of Darkness. The Void Energy creates many things monstrously, but with G'huun the Blood God, a monster was created. He lurks for you in the deep Uldirs, waiting to be released from his allies to the Bloodrolls.
The Halls of Control were created by the Titans as a quarantine facility. They wanted to get to know the Void Energy better to explore the old Gods, but something went terribly wrong and the Void creatures got out of control. The facility was sealed to prevent a great disaster. Three seals must be removed to free the creatures and two seals have already been opened. If things continue as they are, it's only a matter of time before the last seal will also fall.