Complex sentence: The twenty-two Navy SEALs killed were members of the Naval Special Warfare Development Group, more commonly known as SEAL Team Six, the unit that had carried out the operation that killed Osama Bin Laden.
Split into simpler sentences: The twenty-two Navy SEALs killed were members of the Naval Special Warfare Development Group, more commonly known as SEAL Team Six. This unit was best known for its having carried out the operation that killed Osama Bin Laden.