Warning: SPOILERS for X-Men #9.
Jonathan Hickman's take on the likes of Thor. Writer Chris Claremont had simply jotted down a note asking for "miscellaneous alien henchmen" in one scene, and Cockrum just decided to draw the most horrible-looking creature he could think of.
The Brood are Marvel's version of the Xenomorph, in that a host body serves as an incubator for a Brood egg. A Broodling is far more parasitic than the Xenomorph, however, because they release enzymes into the bloodstream of their hosts, gradually mutating them. When the process is finally complete, the host is transformed into a new Brood, retaining all the memories and powers. The Brood have successfully taken over some of Marvel's most powerful and formidable heroes, including the likes of Charles Xavier, Ghost Rider, and even Thor the All-Father.
X-Men #9 has revealed the secret origin of the Brood. It seems the aliens were found spawning in the dark matter flow of a collapsed universe. Although it's not explicitly stated, it's entirely possible they actually originated from that other universe. Left to their own devices, the Brood would have remained in that place, evolving along a seasonal path of factional queens and dueling hives. But they had been discovered by the Kree Empire, and the Kree conceived them as a weapon.
The Kree Supreme Intelligence concluded that the Brood could be used to bring down their rivals. They could introduce the Brood to another empire's space, and the Supreme Intelligence believed they would reproduce beyond the point where they could be kept in check. It would take no less than eight millennia for a galactic empire to fall to the Brood - and the Kree decided to target the Shi'ar with this particular weapon. The Kree then created a genetically engineered weapon called the "King Egg," a control mechanism to help direct the Brood and ensure they didn't ultimately threaten the Kree Empire as well.
This is hardly the first time the Kree have attempted to weaponize other races. These experiments have tended to backfire, however, most notably with the creation of the Inhumans - and Black Bolt, the Midnight King destined to bring the Kree down. It seems there have been other such undertakings, as well; an information page records the Kree attempting to weaponize races such as the Tal Ba-Rii, the Sidri, the Scatter, the Phalanx, and the Gu'Knoiss. Some of these races are long since forgotten, and thus have never been mentioned in Marvel before, while others are notable predators. It's fascinating to now number the Brood among the Kree's failed experiments.
X-Men #9 is on sale now from Marvel Comics.
