WARNING: The following contains spoilers for Doctor Doom #8 by Christopher Cantwell, Salvador Larroca, Guru-eFX, and VC's Cory Petit, as well as spoilers for Marvel Zombies: Resurrection #3 by Phillip Kennedy Johnson, Leonard Kirk, Rachelle Rosenberg, and VC's Travis Lanham, both on sale now.
The X-Men have dealt with more than their fair share of terrifying monsters and extraterrestrial threats. Some of these former adversaries have gone on to become friendly acquaintances and homesteads, a la Krakoa the Living Island, while others have moved on from their original nemeses and onto other prey. On more than one occasion, the X-Men have seen their previous enemies go on to become powerful enough that they threaten the entire world, if not the whole galaxy or beyond. Currently, one of those menaces is waiting in the wings in the primary Marvel universe, while giving more than a glimpse of its true potential in others.
An ancient, alien race, the Brood are an insectoid species with ravenous appetites that serve only their Brood Queen's whims. As a hivemind, they are not incapable of intricate thought, they just don't practice it often, usually focused on savagely ripping through whatever is closest to them. Able to infect other beings and use them as host bodies to incubate their own species, the Brood that emerges from such a process usually carries on some amount of its host's memories and powers, assuming they had them, but the personality of the host is completely lost to the transformation. Most often associated with the X-Men, the Brood have come into contact with the mutant heroes many times throughout their Marvel comics history, though they've never been particularly successful in their endeavors, at least, not in the main Marvel universe.
In Marvel Zombies: Resurrection the Brood have been revealed as the penultimate threat in the universe as the progenitors of the zombie plague. The zombies operated as a hivemind, and the survivors have been searching for the Galactus Hive in order to determine the cause of the outbreak and, with any luck, find a cure for it. After discovering a bound and tortured Logan, Peter Parker, the Reed Siblings, and Blade are ambushed by a zombified Magik who teleports them to Limbo, now known as Blackspring. The zombified Silver Surfer explains that there is no cure because there is no disease. The zombies are an evolutionary offshoot of the Brood, made by implanting Brood eggs inside of the living embodiment of hunger itself -- Galactus. What emerged were Brood parasites unlike any before, with an even greater desire to consume, but without losing any of the intelligence or knowledge of the host bodies they claim. It's not only the most threatening the Brood have ever been, it's quite possibly the greatest threat across the entire Marvel multiverse.
In Doctor Doom #8, Blue Marvel has ended up powerless and stranded inside of a black hole with an alternate universe version of Otto Octavius. While they work on a way out, as well as a solution to the black hole itself, the two are ambushed by the Brood. Coming from the void itself, they make short work of Doctor Octavius, tearing him to shreds and devouring the man entirely. Blue Marvel does everything he can to fight back, and manages to escape the assault before he tumbles into the singularity and across its threshold to someplace as of yet unknown. There's no telling why, or how the Brood were inside the black hole, but even inside of a literal vacuum they are as ferocious as ever.
Currently, the Brood mutant and X-Men ally Broo is the only thing keeping the species in check in the main Marvel universe, only adding to the confusion regarding what happened to Blue Marvel. Thousands of years ago, the Kree developed a King Egg, one that would introduce a patriarchal aspect to the Brood and disrupt their matriarchal hivemind, allowing them to be weaponized and controlled through a single source entity. During the events of Empyre, while the X-Men were being hunted by the Brood for possession of the King Egg, the heroes were chased to a desolate alien planet. While they put up the best fight they could, the Brood were too numerous, and death seemed imminent.
However, the onslaught suddenly ended when Broo found and devoured the King Egg already. As a result, Broo is now able to exert control over the entirety of the Brood, overriding the Queens and ensuring safety, at least while the power he gained from the egg remains in effect. Unfortunately, the effects of the egg are only temporary, so it's only a matter of time until the Brood become a serious problem again, and with what they've already done elsewhere, they are poised to become a major cosmic threat.