Dark Alternate Future

Origin
In March of 2016, Randolph Carter asked Rex Buckland and Hannah Webster to meet him at Miskatonic University. When the two former Warlocks arrived, Randolph used a spell to put them to sleep.

Randolph did this in order to use the Hobus Crystal, a mysterious crystal with the powers to show possible time lines to people as a dream. Randolph then showed Rex and Hannah a time line in which they never existed. In that time line, the Old Ones had escaped their prison and had wiped out all life on Earth and beyond.

History
In this time line, the Old Ones somehow escaped from their empty dimension prison and attacked first the White Lighter Elders and the Demonic Underworld. Both were totally wiped out. The Underworld was subsequently taken over by the Mi-go. What happened to the Realm of the Elders was not made clear.

After that, the Old Ones turned their attention to Earth. They attacked the helpless planet and exterminated the entire human race. Nothing was spared, not even the animals.

Even the forces of magic, including the Charmed Ones, were unable to repel the Old Ones. The Charmed Ones made their final stand in their San Francisco home, where they were all subsequently killed by the Old Ones.

Rex and Hannah in the dark alternate future
Rex and Hannah experienced this possible future thanks to the dream showed to them via the aforementioned Hobus Crystal. The two former Warlocks were horrified to see that the Earth wiped out and that everyone they knew and cared about was now dead.

It didn't take Rex and Hannah long to realize that the Old Ones had been responsible for the destruction of all life on the planet.

At the end, just as Cthulhu himself was attacking them, Rex and Hannah declared their love for each other and vowed that Cthulhu would never break them. It was at that point that Randolph ended the dream and woke both Rex and Hannah up.

Incentive to stop the Old Ones
The reason Randolph showed this alternate future to Rex and Hannah was as an incentive to show them what was at stake if the Old Ones were not prevented from escaping their empty dimension prison.

More to the point, this alternate future only came to pass due to Rex and Hannah having not existed in that time line. This revealed that the two former Warlocks will play a key role in stopping the Old Ones from escaping their prison. However, just when and how this will happen remains a mystery at this time.