If the Spire is upgraded to a Greater Spire, the Mutalisk can morph into a Guardian, a dedicated long-range bomber; and, in, it can turn into a Devourer, which excels at countering enemy air units.
The Mutalisk can morph into a Guardian. The Spire is upgraded to a Greater Spire. A Guardian is a dedicated long-range bomber. It can turn into a Devourer. This is in. A Devourer excels at countering enemy air units.