He-Man/ThunderCats is a comic book limited series. The series features a storyline that crosses-over the Masters of the Universe and ThunderCats franchises.
Plot
After another defeat at the hands of Lion-O Lord of the Thundercats, Mumm-Ra returns to his Black Pyramid to restore himself. After being healed, Mumm-Ra's masters, The Ancient Spirits of Evil claim to have found a weapon that will defeat the Thundercats once and for all, The Power Sword of He-Man.
On Eternia in Eternos city, Prince Adam awakens with a start and races to a ceremony in his honor only to find the assemble heroes, as well as Adam's father King Randor have been waiting for him over a half hour. King Randor berates his son claiming Adam is clearly not ready for the responsibility of taking on royal duties. Suddenly, an earthquake shakes the city and all look up to see Third Earth, home of the Thundercats and Mumm-Ra tearing through space into orbit above Eternia.
Back on Third Earth, the Thundercats are startled by the earthquakes and the appearance of Eternia in their skies. Their long time mentor, Jaga appears in his ghost form and tells them about Eternia and that he fears that both worlds will live or die together. Lion-O raises the Sword of Omens to grant himself sight beyond sight to see what danger the Thundercats now face.
Back on Eternia, Third Earth's presence has caused massive earthquakes that the heroes of Eternia rush to defend the population against. Finally, a massive tidal wave rises up and swamps the city throwing Adam, Randor, and Adam's talking pet tiger Cringer over a balcony and into the waters. Adam manages to save the unconscious Randor and pulls both of them to higher ground with Cringer. As Adam raises the Sword of Power to become He-Man the Sorceress of Grayskull arrives to stop him, telling him that an evil has come for the sword and she will secure it within Castle Grayskull. When Adam hands the sword over the Sorceress stabs him clear through the chest, revealing herself to be Mumm-Ra in disguise.
After confirming their allies on Third Earth are safe from the quakes the Thundercats prepare to head to Eternia. Lion-O speaks his fears about the mission to Panthro, wondering if he, as the Thundercats ruler, is up to the task ahead. Panthro encourages the young king and says both the Sword of Omens and the Thundercats have faith in him. Lion-O thanks his longtime friend as they head to the ship with the others bound for Eternia.
Mumm-Ra gloats as he holds up the sword as Adam remains impaled on it, saying he can sense all evil as well as a lack of evil. Adam's lack of it was the beacon that allowed Mumm-Ra to find him and probe his mind to take a form Adam trusted to give up the sword. Adam manages to grip the sword and summon the power of Grayskull to transform into He-Man, healing his wounds. Mumm-Ra and He-Man battle as Mumm-Ra uses the sword against both He-Man and Cringer. Just as He-Man gets the upper hand Mumm-Ra vanishes in a flash of light. He reappears in a dark corridor and hears the Ancient Spirits of Evil speaking to someone. The Spirits say that both the sword and Mumm-Ra are his to do with what he pleases as long as he fulfills his end of the bargain. Before Mumm-Ra can react he is blasted to dust and Skeletor reaches down and picks up the Power Sword telling the Spirits they have a deal.
Skeletor retreats to his lab with Mumm-Ra's helmet full of ashes and the Sword of Power and begins working. He quickly finds that, despite his condition, Mumm-Ra is still alive. Amused, Skeletor tells Mumm-Ra that he found out about the Spirits through the Golden Disks of Knowledge and struck a deal with them: the Power Sword and Mumm-Ra in exchange for freedom from the Black Pyramid, something Mumm-Ra was never able to do. He-Man and Cringer tend to King Randor who suddenly awakens in a panic worried about Adam. He-Man assures Randor that Adam is safe and that Adam is just as worried about him. Suddenly both men are attacked by the combine forces of both the Mutants of Third Earth and Skeletor's minions. After a furious battle that sees He-Man overcoming Evil Lynn's magic, the Mutants weapons, and physical attacks on all sides, He-Man defeats them all with help from Randor.
Back in Skeletor's lair at Snake Mountain he explains to Mumm-Ra that he could never wield the Power of Grayskull without it consuming him. But by mixing Mumm-Ra's ashes to a potion it will link Mumm-Ra's ever living power to him allowing him to use the Power Sword. Despite Mumm-Ra's protests Skeletor drinks the potion and summons the Power of Grayskull to him adding He-Man's strength to his own. He-Man suddenly is transformed back into Prince Adam and collapses in Randor's arms with the mortal wound still in his chest.
Skeletor revels in his newfound power saying that despite the fact he could not have gotten it without the Spirits help, he has no intention of freeing them from the Pyramid. Snake Mountain is suddenly rocked by an attack and a wall explodes in front of Skeletor, revealing the full team of Thundercats with Lion-O leading the charge.
Skeletor's strength initially pushes the Thundercats back and the battle is joined. All of them take turns in attacking Skeletor who laughs off the attempts saying they have no idea the power he now wields. Lion-O attacks and manages to wound Skeletor while Skeletor slashes Lion-O's eye with the Power Sword.
In the midst of battle Mumm-Ra tries to assert dominance now he, in ash form, has been bonded to Skeletor. Skeletor tells Mumm-Ra to back off as he is fighting Mumm-Ra's mortal enemies but it is too late. Lion-O realizes the two are bonded and uses Mumm-Ra's weakness against his own reflection to drive Skeletor off and retrieve the Power Sword.
Skeletor manages to teleport away trying to regain control from Mumm-Ra and the two fight an internal battle for their now shared form. Mumm-Ra manages to take command and reassume his original form but in doing so loses Skeletor the power he received from the sword. The two quickly realize they are too evenly matched and bonded thanks to the potion and Skeletor says he has a way they can both get what they want. Lion-O and the Thundercats arrive at the damaged Eternos to find all the heroes in morning as Adam has died from his wounds. Shocked that he was unable to return the Power Sword to its rightful owner Lion-O speaks alone with King Randor. Lion-O says he had heard Randor's son was a great hero and Randor says he didn't know Adam was He-Man and is only now realizing himself what a hero Adam was. Lion-O tells Randor that despite not knowing he is sure Adam felt the love of a father from him. Randor thanks Lion-O and tells him in turn not to worry about living up to anyone's ideals but to be a ruler from his own heart.
The city is suddenly rocked by another earthquake and everyone present looks up to find four massive giants attacking the city. Mumm-Ra and Skeletor have used their combined power and knowledge to free the Ancient Spirits of Evil and take control of them and now have set them loose to attack Eternia and take the power of Castle Grayskull.
Lion-O pledges the Thundercats aid to help Randor fight off the giants as the two different groups banter. Lion-O sees the Mutant leader Slithe brought in after his defeat at He-Man's hands and the Mutant mocks Lion-O saying he will never beat Mumm-Ra because he can come back from death time and again.
Slithe's comment gives Lion-O an idea and he orders the Thundercats to help Randor and Eternia's heroes defend Grayskull. He then takes Adam's body and flies back to Third Earth with Cringer, who refuses to leave Adam's side. They arrive at Mumm-Ra's Black Pyramid and find that some of the evil warriors who attacked He-Man retreated here to loot Mumm-Ra's treasures while he is away. Lion-O and Cringer manage to get Adam's body and the Sword of Power into Mumm-Ra's sarcophagus in the hopes it will bring Adam back as it has Mumm-Ra. When nothing happens at first Lion-O tells Cringer to check on the villains looting the Pyramid and uses the Sword of Omens power to transform Cringer into a version of his Battle Cat form.
Now alone, Lion-O prays his efforts will work and then hears a scratching inside the sarcophagus. He hears a whisper of Adam calling the power of Grayskull and the sarcophagus explodes and an enraged He-Man comes out and attacks Lion-O. Realizing the combination of power has healed He-Man but also driven him insane, Lion-O defends himself with the Sword of Omens and the two heroes clash. Lion-O separates He-Man from the Power Sword but He-Man is too strong and nearly drowns Lion-O in Mumm-Ra's mystic pool before the Thundercat can escape. Realizing he has to increase his own power to survive Lion-O calls for the Sword of Omens to give him Power Beyond Power. Now more matched in strength Lion-O manages to wrestle He-Man from behind and put the Sword of Omens in front of him, giving He-Man sight beyond sight. Seeing a horrifying future vision where Mumm-Ra and Skeletor have killed or enslaved everything snaps He-Man out of his madness and he thanks Lion-O as the two introduce themselves.
The two heroes find Cringer, transformed back to his normal form, happily licking his paw over the defeated villains and Cringer runs into He-Man's arms thrilled his best friend is alive. The three rush back to the ship desperate to get back to Eternia before it is too late. The four Ancient Spirits of Evil have arrived on the doorstep of Castle Grayskull while Skeletor and Mumm-Ra look on. Randor has assembled an army alongside the Thundercats and the combined heroes attack the giants. Man-At-Arms, Tygra, and Panthro assemble a cannon of combined Eternian and Thundarian tech to attack the Spirits while the others buy them time.
Inside the Castle the Sorceress summons the Castle's defenses to shield it from attack when Jaga's spirit form arrives. Jaga tells the Sorceress that he believes there is a way to stop the Spirits but the answer lies on the Thundercat's destroyed home world of Thundara. The Sorceress opens a portal in space and time for Jaga and her to slip through and get the item they need. Outside, despite the hero's best efforts, the Spirits close in on Castle Grayskull. When Man-At-Arms, Tygra, and Panthro fire the cannon at one of the Spirit's head it simply laughs off the attempt and turns to attack the castle. Tygra then uses his powers of illusion to make Grayskull seem to disappear much to the Spirit's confusion and Mumm-Ra and Skeletor's frustration.
Jaga and the Sorceress appear to the heroes and tell them that Jaga knew of a Golden Disk of Knowledge among the treasures of Thundara. The two use the retrieved Disk to expose that Skeletor and Mumm-Ra used Skeletor's Disks of Knowledge to bring the Spirits to physical form and each of the four Spirits have a Disk on their chest which Jaga and the Sorceress now expose. If the heroes destroy the Disks the Spirits will be defeated. Teela and Cheetara take a flyer and attack one of the Spirits and are saved when Cheetara's sixth sense allows them to avoid an energy blast. Teela sets the flyer to crash into the disk and as both escape the first Spirit is destroyed. Orko manages to accidentally enlarge Snarf, who hits the Disk on the second Spirit, stopping it as well.
Wilikit and Wilicat join Buzz-Off and Stratos to attack the third Spirit and after Buzz-Off and Stratos are injured in the distraction WiliKit and Kat break the Disk stopping the third Spirit.