A year ago, I asked on the possibility of Teller Nuclear Bombs, where the bomb is hot enough to fuse oxygen and cause a runway reaction that destroys Earth by using the air as "fusion material". This type of world destruction is especially common in WH40k, which we all know as Exterminatus.
Of course, in that thread, it was dismissed as being impossible due to the insanely high temperatures required. As stated on wikipedia regarding the Oxygen-Burning Process, the temperature required for such fusion processes would be around 1 to 2 billion degrees.
But now, I decided to change the rules. I no longer need modern day technology. Assume this is the far future. Feel free to use any unobtainium (Degenerate or Exotic Matter, Micro Black Holes, Cosmic Rays) to see if the bomb has any possibility of coming to be. As long as it is not utter handwavium (Physics-defying matter that isn't even deemed as Dark Matter or Energy), it is all fine.
PS: I would ask this because even with said unobtainium, I do not know what could cause such an event to happen.