Halo 3 - Thunderstorm Achievement

Find and claim this skull on Normal difficulty or Harder

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After you shut down tower 1 and get access to the hornet, fly to tower 2 (the one the arbiter shut down). While walking up the stairs, go to the middle part that connects both and there will be a stair leading up to a platform where the skull is.
Pick it up as if it were a regular weapon, and the achievement will unlock.

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All skulls location video by RoosterTeeth