The Greater Spire is morphed from the Zerg Spire and requires a Hive to be morphed. It allows the Zerg player's Mutalisks to morph into Guardians and Devourers. The Greater Spire does not build any units directly.
While Guardians are occasionally used against Terran (most often against M&M builds) and to harass, Devourers are very rarely used although they have a place in extreme late-game air-air armies.
The Greater Spire also allows the research of two advanced upgrades beyond the +1 level: Flyer Carapace for air unit armor and Flyer Attack for air unit attack.