He-Man/ThunderCats was a comic book crossover that was released between 2016 and 2017.
Official DC synopsis
He-Man and the Masters of the Universe team up with the ThunderCats—the epic crossover event you've waited thirty years to see! In his ever-living desire to destroy the mighty ThunderCats, Mumm-Ra quests for a weapon that can rival the legendary Sword of Omens: He-Man's Sword of Power! But his dimension-spanning scheme kick starts a cataclysmic crisis that will embroil heroes and villains—Masters, Mutants and ThunderCats—in a mind-blowing six-part saga!
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.