The last and most mysterious purple felcycle mount has been found! 
Voidfire Deathcycle will drop from Horrific Visions Revisited in Patch 11.1.5.
Specifically, the mount currently lists both Vision of Orgrimmar and Vision of Stormwind Revisited as the main source.
While we know it comes from Horrific Visions, the specific requirement remains to be seen.
Here is what the model looks like in the game:
