The Jormungand Brood was one of the major broods of the Zerg Swarm. It was one of the most heavy-handed zerg attack forces. Typically called on to follow the fast preemptive strikes of the Garm Brood, the Jormungand Brood relied on its massive numbers and brute force to overcome any foes, pacifying all other resistance to the Swarm. It excelled at ground-based combat and direct unit-to-unit warfare.
The Jormungand Brood was the first brood to engage in open warfare with the Terran Confederacy and for all intents and purposes, the terran species as well. In November 2499, the Jormungand Brood force attacked Chau Sara, specifically an Alpha Squadron force sent to deal with the Sons of Korhal. The brood also attacked the Flannum Installation and secured the area around it. Both times however, Alpha Squadron, with the aid of Cerberus Recon Squad, was able to prevail against the brood.
The Jormungand Brood also laid siege to Los Andares and maintained the zerg's primary hive cluster on the planet alongside the Tiamat Brood. While Los Andares was liberated and the primary hive cluster destroyed, these were minor setbacks. The Confederacy placed Chau Sara under a lockdown, allowing the brood to continue to operate on the planet.
Prior to the battle at Mar Sara, several groups of zerglings of Jormungand Brood were encountered by marines onboard a space platform orbiting the planet during an equipment demonstration for the new colonial magistrate. They were not identified as zerg at the time.
Elements of the Jormungand Brood were present on a space platform above Mar Sara but its most notable deployment after Chau Sara was on Tarsonis. The Jormungand Brood established their primary hive cluster in and around New Gettysburg, attacking the Confederates. When the protoss arrived to attack the zerg, Arcturus Mengsk, fearing the Confederates might escape, sent Sarah Kerrigan and a contingent of Sons of Korhal troops to defend the zerg and neutralize the protoss. After the protoss were destroyed, a massive swarm of zerg overran Kerrigan's base camp and destroyed its defenders, abducting Kerrigan herself. Kerrigan was put in a chrysalis during a long process of infestation, and a new brood was formed to protect the chrysalis as it matured.
The Jormungand Brood played a key role in the defense of the Overmind, fighting alongside the Tiamat Brood. Despite the joint defense, a combined protoss/terran force succeeded in overcoming the defenses and destroying the Overmind. In the aftermath of the Overmind's destruction, the brood's minions roamed Aiur like mindless beasts, removed from the Swarm's power structure.
The Jormungand Brood is named after Jörmungandr, a giant serpent in Norse mythology that is so large that it circles the world and bites its own tail. It will be killed by Thor, who will fall dead after nine steps from Jörmungandr's poison.