Apocalypse World

An Alternative Universe that could only come about had Mary Campbell refused to make a deal with Azazel, which ensured her boyfriend John Winchester remained dead and their future sons, Sam and Dean Winchester, were never born. It is one of the multiple Alternate Universes shown in Supernatural.

History
After John Winchester is killed by Prince of Hell Azazel, Mary refuses to make a deal to bring John back. Mary remains alone and a Campbell for the rest of her life, rendering her future, pre-destined sons Sam and Dean Winchester as non-existent.

In this timeline, Heaven and Hell go head to head in a war, almost eliminating the entire human race.

By 2017, one of the surviving humans is Bobby Singer, who is equipped with angel-killing bullets. In this universe, a class of demons known as Tempter demons exist.

Season 12
While his mother Kelly Kline goes into labor, the Nephilim Jack opens a portal that leads to this alternate universe. His protector Castiel falls into it and meets Bobby Singer there. He manages to escape and show this world to Sam and Dean. When Lucifer arrives in hopes of claiming his son, Sam, Dean, Castiel and Crowley decide to use this portal as a means to lock Lucifer away.

Crowley chooses to sacrifice himself as part of a ritual that will seal the rift. Unfortunately, Lucifer manages to escape and kill Castiel. He is forced back into the rift when Mary Winchester attacks him and both end up trapped.

Aftermath
The state of this world encourages Sam and Dean to believe that they are indeed heroes who have saved the world, preventing it from becoming what it did in that universe. The major downside is Mary's absence, as well as Castiel's death.

Appearances

 * Season 12
 * All Along the Watchtower

Trivia

 * This universe is similar to the alternate universe of 2014, in which Sam said "yes" to Lucifer, allowing the Archangel to use his True Vessel to exterminate the human population. The main difference is that the latter is a parallel reality while the universe of 2014 is an alternate timeline that never happened.
 * This is the third known alternate universe on the show with the one in The End and The French Mistake being the first two.