The Starcursed Voidstrider is an Alliance-only mount that can be obtained by completing the Void Elf recruitment scenario. The recruitment scenario itself is pretty straight forward, but you also need to complete the following requirements before you can begin it:
- Be level 110.
- Complete the "You are now prepared!" achievement. This requires you do complete all of the Argus campaign.
- Reach Exalted with Argussian Reach. This is a faction located on Argus, and you will need to do a lot of World Quests to get to exalted with them.
Once you have completed these requirements, head to Stormwind Embassy to begin the recruitment scenario.