Captain Marvel will formally introduce the shapeshifting Skrulls into the MCU, but an official tie-in book has already given a sense of their weaknesses. The Skrulls are a militant race who have been locked in an eons-long war with the Kree Empire, and by all accounts, they've been successfully occupying the outskirts of Kree space.

Loosely inspired by the classic comic book story "The Kree-Skrull War," Captain Marvel will see Earth become a galactic No-Man's Land in the war between these two alien empires. The Skrulls have already infiltrated Earth's defenses, with their leader Talos taking up the position of a high-ranking S.H.I.E.L.D. agent. Fortunately for humanity, Captain Marvel comes crashing down to Earth and soon teams up with the young Nick Fury and Phil Coulson.

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Steve Behling's official tie-in novel Starforce on the Rise finally reveals some of the Skrulls' weaknesses. Firstly, it confirms that a Skrull's body returns to its natural form when it dies. That particular limitation is lifted straight from the comics, where the Avengers only discovered a Skrull invasion after the death of the Skrull who had replaced the Hand leader Elektra. This is quite an important detail; it explains how S.H.I.E.L.D. acquired a Skrull body for postmortem in the Captain Marvel trailers. Presumably, the Skrull was killed while impersonating a human and reverted back to its natural form.

But the Skrulls do have other limitations. It seems that, over the course of their millennia-long war with the shapeshifters, the Kree have developed the technology to detect them. In Starforce on the Rise, Kree agents are equipped with life-form detectors that identify whether or not anyone has been replaced by a Skrull. These detectors can be jammed, but ironically by doing this the Skrulls would tip the Kree off to the presence of at least one infiltrator in their midst. Again, this could be key to the plot of Captain Marvel; it would explain why Carol Danvers is able to identify Kree impostors, and perhaps even why she's so confident that sweet little old lady is really a Skrull.

A lot of fans believe that Captain Marvel will set up a "Secret Invasion" plotline that will run through the MCU's Phase 4. If that is indeed the case, the limits to the Skrulls' shapeshifting powers could be crucial to stopping them. The heroes could potentially be tipped off to the invasion if one of their own is killed in action, and reverts to Skrull form; meanwhile, in theory, Captain Marvel could help them put together their own life-form detectors and identify further impostors.

More: How & Why Marvel's Phase 3 Slate Changed So Much