The "Voidtalon of the Dark Star" is a new mount added to WoW with patch 6.1, but the patch notes don't mention anything about how to obtain it. A few days have passed since the update and people are finally figuring out the process. It's not that easy.
The official survival guide only says that you have to be lucky to find it, and that's 100% true. The mount is obtained by clicking on purple portals called "Edge of Reality", which spawn in level 100 areas in Draenor for a short period of time, and take you to a mini-scenario.
This is what the portal looks like. Some sort of misty gateway:
And this is where their spawning locations are. Or at least these are the ones people have found so far. As you can see they're all around the "Apexis daily" areas, which means it's always crowded. So yeah you have to be lucky to be the first one finding the portal AND be able to click it in time, since they close after a few minutes even if no one clicked on them.
Also, you can't enter the portal if you already have the mount or if you're in a premade group.
Once you're inside, you have to loot the eggs of a dead Voidtalon and get out. No riddles, difficult fights or level 100 elites.
Good luck with all that camping... Or you can come back two expansions later to an empty Draenor without any competition.