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The Brood first show up in Uncanny X-Men #155, during the trouble involving Lilandra, Deathbird, and the Shi'ar. That puts them in the middle of a much bigger space story almost immediately. By the time the main Brood Saga gets going, the X-Men are dealing with something that can threaten them even after a fight appears to be over.
Their reproduction depends on putting embryos inside other living beings. The host can still look like themselves while carrying something that is going to change their body and take over their life. The Brood have queens and warriors, and the queens are central to the way their colonies work.
They also have enough intelligence and organization to make plans, use technology, and exploit other species. They're capable of treating a living person as useful material. The X-Men's powers give the Brood a reason to keep them alive long enough for something worse to happen.
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· Comics / Marvel / X-Men