Medivh's hourglass

Medivh's hourglass (or obsidian hourglass) informally refers to refers to as "an hourglass" then "the glass" and "the hourglass".

Medivh is described giving to Stormwind in the Warcraft: Orcs & Humans manual. It is first mentioned by Sir Lothar in his Chronicles of the War in Azeroth in the year 557:
 * From within his flowing cloak he produced an hourglass, crafted of deepest obsidian, with sands as white as undriven snow.
 * The young prince looked closely, but although the sand seemed to constantly sift from top to bottom, the lower half never filled, and the top never emptied.
 * Medivh claimed that these sands represented the people of the kingdom, and so long as the glass never emptied, the reign of King Wrynn would not fail.

It is mentioned again in the entry for 583 after Azeroth experiences six years of the land sickening:
 * During a bleak morning, Prince Llane rushed to his father's side, carrying the hourglass.
 * During the night, the sands had run down from the top, and it was near emptied.
 * King Wrynn took the glass into his hands, and a chill ran through the very core of his being.
 * As the last sands trickled to the bottom of the glass, a great crashing sound was heard at the gates of Stormwind Keep.

The hourglass is not mentioned again and it's unclear what happened to it. It appears to be foreshadowing that Medivh had already set things in motion (creating the tiny rift to Draenor which would be widened into a small portal over years) when he attended Llane's ceremony and bequeathed the hourglass to the Wrynn family.