Vidmaster Challenge: Brainpan
From Halopedia, the Halo wiki
Vidmaster Challenge: Brainpan
Vidmaster Challenge: Brainpan is one of several achievements in the list of Vidmaster Challenges released in the Title Update 2 for Halo 3. For this achievement you must find all the hidden Skulls on the Mythic maps. The skulls appear the same as in campaign, a human skull.
This particular Vidmaster achievement is worth 25 gamerscore, unlike the other Vidmaster Achievements, which are worth zero gamerscore. You will also get Recon Armor if you get all the Vidmaster Challenges[1], but you must first get all the Vidmaster challenges included with Halo 3: ODST.
Trivia
Look at the background of the picture and you will see the Marathon symbol which appears on all of the Vidmaster achievements.
Sources