There's a steep grind you'll have to take part in if you want to legitimately trigger GTA Online's Chiliad mystery mission, but it's not as bad as what we once thought.
A couple of weeks ago, after years of being dormant, the mount Chiliad mystery was revived thanks to modding/hacking group The Gurus. The team managed to find traces of a mission related to the Chiliad mystery in the most recent GTA Online update: Gunrunning.
After some careful adjustments to game values, a hidden mission called Alien Egg Supply Run was discovered, and everyone cheered that the mystery had been solved. However, at the time, The Gurus hadn't actually figured out how to trigger the mission legitimately.
The modding team then announced how those who don't want to mess with game files can legitimately trigger and play the mission in GTA Online. The team believed you would needed to complete 600 Gunrunning sales to trigger the Alien Egg mission.
This is in fact not correct. What you need is 600 Resupply runs, which are actually easier and less time-consuming than doing 600 sales. After that's done, you'll still need to start another Resupply run between 1:00 and 23:00 hours.
The mission can only be triggered once. So, if you have the patience for this grind and manage to eventually unlock it, you'll only be able to play it one time. If you don't intend on doing this, you can watch the video above from GTA Series Videos showcasing the full thing.
The thrill was in trying to figure out how to trigger it, after all, not so much playing it.