The following contains spoilers for Wolverine #29 and Legion of X #9, now on sale from Marvel Comics.
The X-Men have turned all sorts of threats and foes into allies in the modern era. One of the most useful has been the development of Krakoa -- originally a threat to the X-Men -- into becoming an ally to the mutants. Marvel's oldest mutant has served as the X-Men's base of operations and greatest natural resource in the modern Krakoa Era, but all that strength might be corrupted into a very deadly weakness.
The events of Wolverine #29 (by Benjamin Percy, Juan Jose Ryp, Frank D' Armata, and VC's Cory Petit) and Legion of X #9 (by Si Spurrier, Netho Diaz, Sean Parsons, Álvaro López, Java Tartaglia, and VC's Clayton Cowles) reveal that Sabretooth and Nimrod have both found ways to imprint themselves onto Krakoa and corrupt the ancient mutant. This is a serious blow to the X-Men - and even if they survive the chaos caused by these revelations, it could open the door for even greater attacks against the mutant nation.
How Krakoa Gained A Sabretooth Form
In Wolverine #29, Logan finds himself trapped within the Pit of Krakoa. The mutant nation's prison traps someone within a mental construct, as witnessed by Victor Creed and plenty of other mutants during the events of Sabretooth. But it seems that Creed left a serious impact on Krakoa even after his departure. A piece of corrupted bio-code within the ancient mutant resulted in an imprint of Sabretooth remaining within the Pit. This "Sabretooth Virus" means that Krakoa has a Sabretooth shade causing chaos within it, giving part of the oldest mutant a sadistic and vicious side. While X-Force identified the problem, Wolverine's time in the Pit put him face-to-face with this remnant of Victor Creed. Luckily, Logan was able to defeat the avatar with the same bio-organic technology Beast had been previously using to control him and allowed him to escape the Pit.
It's a win for Wolverine, and potentially deals with the infection once and for all. But given Krakoa and Sabretooth's ability to regenerate from almost any blow, the Sabretooth Virus may yet remain a part of Krakoa. This could mean that a piece of Sabretooth will live on even beyond what the nominally immortal mutant could ever hope to achieve. Given Krakoa's status as one of Marvel's oldest and most indestructible mutants, this could allow a piece of Sabretooth to live on forever. It's a terrible revelation given that other mutants may remain trapped in the Pit and find themselves subject to Sabretooth's darker impulses filtered through Krakoa's powers. But it's not the most horrifying corruption of Krakoa that an X-Men enemy has found use for.
How Nimrod Corrupts Krakoa To Create New Sentinels
Legion of X #9 reveals that the mystical assault on the X-Men that's been corrupting Angel and Nightcrawler was orchestrated by Orchis. The anti-mutant organization wanted to disrupt the mutants ahead of a major attack that thanks to the Astral Technarchy has been targeting the Alter. The Super-Sentinel known as Nimrod was able to quietly breach the Alter and take a certain level of control over Warlock -- allowing the mechanized menace to in-turn target Warlock's connection to Krakoa. A plan initially centered on observation and subterfuge all of a sudden became an opening for a full-scale attack, which Nimrod has taken full advantage of. Through Warlock, Nimrod is able to effectively take over Krakoa and target countless mutants across the nation. This turns many sleeping mutants into agents of Nimrod who quickly attack the uninfected - and allows Nimrod to grow entire Krakoa-Sentinels to further push the sudden assault.
In one fell swoop, Nimrod has found a way to turn the X-Men's greatest asset into a serious threat to every mutant present. Furthermore, Nimrod's corruption could seriously weaken the mutant nation at a very inopportune time, and may even be an early indicator to Orchis that it's time to carry out the kind of attacks that will fuel the upcoming Fall of X storyline. The corruption of Krakoa itself is a major blow to the X-Men, as it could disrupt things like the Krakoa Gates and completely unravel the resurrection protocols. It also keeps Krakoa itself from getting involved in the conflict, which could be a game changer for the X-Men. By targeting Krakoa itself instead of just the nation it shares a name with, the X-Men's enemies may have found the opening they need to break the mutant's most successful period.