Here’s how to get the secret mission “Piercing the Shroud” in Starcraft 2.
In the mission “Media Blitz” go to the bottom right corner of the map. There you will see a red civilian building. Destroy it and pickup the secret documents that will drop after it’s destroyed. The new mission will now show up on your star map back on the Hyperion bridge. I’ve heard tales that you cannot play this mission after you go to Char so if that is the case make sure you load up an earlier save! I won’t let you know what the mission is about so as not to spoil it but be sure to check it out!
Steel Nutz is an avid pc gamer and custom rig building enthusiast with a love for RTS, MMO, and FPS genres. He is also the founder and creator of PC Gamer FTW and FTW Networks. 
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You also have to complete the mission on Media Blitz once you’ve collected the Secret Documents in order to have Piercing the Shroud showing up in the Star Map. Also, it does seem that you cannot play it once you’ve gone to Char. I had to load up a past save, do the mission for the achievements, and then go back. But you cannot complete all 26 missions that way for the achievement. You must go back before you go to char, complete all the missions, and then head to char in order to get 26/26 missions completed.
Thanks for the info Taylor!