WARNING: The following contains spoilers for Legends of the Dark Knight #10 by Brandon Thomas, Giannis Milonogiannis, Jordie Bellaire and Deron Bennett, on sale now.
Legends of the Dark Knight #10 sees Batman's Gotham City and Justice League lives collide in a disturbing way. Usually, the high-stakes intergalactic threats of the League's business don't often converge with Gotham's criminally insane. However, one Gotham villain has changed that by gaining the powers of one of the Justice League's most powerful members, the Martian Manhunter.
The previous issue saw Batman bring in J'onn J'onzz, the Martain Manhunter, to consult on a case. After a blimp explosion spelled out a Martian letter, Batman deduced that Calendar Man was now using a Martian calendar to conduct his crimes. Batman called in Martian Manhunter to learn more details about this particular calendar, which was rumored to have magic spells hidden within it.
As the two investigators team up, Legends of the Dark Knight #10 opens with some good old-fashioned detective work, as Batman takes J'onn to two other Calendar Man crime scenes. Manhunter is able to identify two more Martian letters, one at each crime scene, which Batman missed. Both crimes coincide with two Martian holidays. Though only a ghost story, Manhunter reveals that making three offerings of horrendous acts on consecutive holidays, each worse than the last, is said to grant an audience with an ancient Martian goddess.
The two heroes realize that they have until midnight to stop Calendar Man from committing a third crime to get his fabled audience. They arrive with seconds to spare but soon discover their efforts were in vain. Despite his third attempt failing, the Martian deity appreciated Calendar Man's efforts and gifted him all the powers of a true Martian, making him just as strong as Martian Manhunter.
All Martians have a broad powerset. They possess the usual powers of strength, flight and increased durability, as well as shapeshifting and telepathy. Manhunter, knowing that Calendar Man couldn't know of the telepathy yet, was able to temporarily block him from accessing that power. Still, that leaves the two to fend off a superpowered shapeshifter, which is no easy task.
Calendar Man primarily uses his new shapeshifting abilities to turn into a more terrifying version of himself and keep the heroes at arm's length, which can be any length when shapeshifting is involved. Though Calendar Man believes he has the upper hand already, Batman has years of experience when it comes to Martian physiology thanks to his time with J'onn.
Batman is well aware of Martian weaknesses. This was made abundantly clear during the JLA story "Tower of Babel" by Mark Waid and Howard Porter. This story saw Batman's secret contingency plans for taking down every member of the Justice League stolen and used by Ra's al Ghul. For Martian Manhunter, this took the form of firing nanobots at the Martian which turned his skin into magnesium. Upon contact with the air, the magnesium ignited, causing Manhunter to burst into flames. Fire takes away Martian powers and severely weakens them and can even be fatal.
It seems that Batman was prepared for the nightmare scenario he finds himself in here, having worn special fire-making gauntlets to confront Calendar Man. Manhunter, using telepathy, already knows what Batman has planned and, despite the risks to himself, approves. Batman's gauntlets ignite, now engulfed in flame. The Dark Knight makes short work of Calendar Man, needing only a few punches to take him out.
Although the Calendar Man was defeated, the Martian Manhunter explained that he will retain his powers on every Martian holiday for a whole year. They commit to coming up with countermeasures to these newfound powers before the next holiday. Clearly, Batman already has some tricks up his sleeve when it comes to Martians. "Tower of Babel" showed Batman could probably take Martian Calendar Man down on his own with enough prep teim. Nevertheless, Batman and Martian Manhunter make a deal with the superpowered Calendar Man together in the future, bringing two fo the Justice League's sharpest minds together.