The Swarm Host is a strain of Zerg purposed to be a siege unit. The Swarm Host can inherently burrow. Classified as a siege unit, it endlessly spawns waves of Locusts while burrowed: each wave consists of two Locusts, and these are produced every 17-24 seconds. This allows the Zerg to better besiege entrenched enemy positions. These Locusts hatch from eggs.
In the game series, the Swarm Host is a concept idea for a new class of Zerg ground support unit.
For more information see the Swarm Host on the parent site.