Vision of Time

The Vision of Time is an hourglass created by Kairozdormu with the help of adventurers. The sands within are formed from epoch stones, condensed essences of time found on the Timeless Isle, and with grains of the Sands of Time contained in the Hourglass of Time. Using the Vision of Time, one can witness the past and future, and even alternate timelines.

Mists of Pandaria
Kairoz tasks Adventurers with powering the Vision of Time by gathering epoch stones across the Timeless Isle. Once the Vision of Time is powered, he sends the heroes to key locations in Azeroth to use its powers and try to find a traitor within the Bronze dragonflight. The final vision received is of Soridormi, consort to Nozdormu, dying and Kairoz himself walking away from her corpse.

War Crimes
The Vision of Time is used during the trial of Garrosh Hellscream to show the court exactly what occurred without relying on witness testimony, such as Durotan's attack on Telmor or Malkorok beating Kor'jus within an inch of his life. Kairoz also tinkers with the hourglass to make it stronger, to the point that his fellow dragon Chromie grows suspicious.

When his work on the Vision of Time is complete, Kairoz locks Chromie in a cage and uses the hourglass to escape with Garrosh through history, traveling to the planet thirty-five years before the present day. Kairoz also uses its powers to call on alternate versions of heroes from many different timelines, causing chaos to erupt in the courtroom and distracting anyone from following him and Garrosh.

The Vision of Time was destroyed in the process; Chromie quickly recovered the broken pieces for safekeeping.

Hellscream
While most of the Vision of Time's pieces remained on Azeroth, Kairoz was able to bring one shard of glass to the alternate Draenor along with him. He explained to Garrosh that it was still connected to the other pieces on Azeroth, and could be used to bring them and Garrosh's new Horde back there. Kairoz gave the shard to Garrosh, believing he was incapable of wielding its power. Garrosh instead stabbed Kairoz in the back with the shard, killing him.

Garrosh traveled to Grommashar and met with. Proving himself in a duel against four orcs, Garrosh revealed the Vision's shard and brought Grommash to the Stones of Prophecy. Using the shard, he showed Grommash the events of the main timeline and the enslavement of the orcs to the Burning Legion. Grommash, convinced that this must not be, formed the Iron Horde.

It is unclear what happened to the shard after this, though it ultimately ended up in the hands of. He apparently used it to access the Dark Portal at some stage.

Battle for Azeroth
One of the fragments of the Vision ended up in Eitrigg's possession and used by the Horde to travel back to alternate Draenor in order to recruit the Mag'har orcs during the Fourth War. With the aid of the Bronze dragonflight and Chief Telemancer Oculeth, a champion of the Horde traveled together with Eitrigg to Draenor. They found themselves several years after Archimonde and had been defeated. In this turn of events, the orcs were at war with the draenei, who sought to subjugate them under the orders from a being called the Light Mother. With the Vision, several Mag'har, along with their gronn and ogre servants were brought to Azeroth to serve the Horde. Unintentionally, several botani and saberon were also brought to from their world and managed to flee into the Northern Barrens.

World of Warcraft
The Vision of Time is provided by Kairoz for the following quests:
 * 1) A Vision in Time
 * 2) Refining The Vision
 * 3) Seeking Fate
 * 4) Hidden Threads
 * 5) Courting Destiny
 * 6) One Final Turn

Speculation
While not directly confirmed, it is very likely that Garrosh indeed used his shard to link the alternate world with Azeroth through the as Kairoz had suggested.