Skull Ops can also be accessed from the Radar. They are marked by a Skull icon on the enemy Avatar, and will be moved automatically to the top of your Operations list.
Completing Offensive or Defensive Special Ops will unlock Skull Ops. Skull Ops are more challenging than regular Special Ops, and will require greater force to complete. However, Skull Ops will provide more valuable Rewards - such as powerful Units, Elite Points, and Experience Points.