During his lifetime, Mū taught Ōnoki how to use Dust Release. At some point in time, both he and Ōnoki met with Madara Uchiha, after an alliance had been formed between their respective villages. However, despite their standing, Madara boldly declared that Konohagakure remained the dominant power and as such, they should obey any orders given. This culminated in both parties engaging in battle where Mū and Ōnoki were defeated.
He also had a great enmity with the Second Mizukage and the two men eventually killed each other in what would be their final confrontation.
Mū summons the other Kage
In preparation for the Fourth Shinobi World War, Mū was reincarnated by Kabuto Yakushi alongside three other deceased Kage to fight against the Allied Shinobi Forces. When they were mobilised, Mū was sent in the direction of the Fourth Division but as he approached their location invisibly, he is detected by Gaara — a feat which he commended.
Upon being noticed, Mū was forced to summon the Third Raikage, the Second Mizukage and the Fourth Kazekage. After a brief argument with the Mizukage over the manner of their deaths, Mū noted the means of their reincarnation, and searched for the Second Hokage, the technique's inventor and person he presumed was using the technique, but later informed his fellow Kage that he could only pick up the presence of someone whose chakra signature was similar to that of the Fourth Kazekage's and that, that person was backed by a large army. The Kazekage then explained that Tobirama had been dead for many years while he was alive and then declared that another also knew how to use the deceased Hokage's technique as well. Mū later notified the Mizukage that they were being controlled and thus, some of their movements were restricted, much to the latter — who wanted to kill him — and the Raikage's — who was looking forward to battling them all — dismay.
Forced to pursue the division throughout the night alongside the others, he remarked that their indecisive targets had been slowly retreating but that the opposition's movements had now altered, concluding their pursuit. When the Mizukage lamented the prospect of confronting shinobi from his village and openly cursed the one responsible for reincarnating them, even Mū shared his sentiments. He then detected two individuals approaching, who are preceded by a colossal wave of sand that bore down on the Kage, but the Fourth Kazekage swiftly counteracted this with his Gold Dust. At this point, Gaara appeared suspended above them and sculpted hands of sand to ensnare the Kage, with only Mū able to escape captivity while sensing Ōnoki's presence above them. Greeting his old pupil he noted that Ōnoki had lived a long life thanks in part to him passing down his teachings to him. Utilising the same technique to repel that of his former disciple, the converging attacks resulted in a huge crater in the middle of the battlefield, leading Ōnoki to assert that his predecessor's strength hadn't waned.
Ōnoki and Mū's Great Battle
Suspecting a shinobi alliance had been formed due to the accumulation of chakra types from different villages, Mū was even more surprised that such a merger has occurred when it was confirmed and even more so that Ōnoki had joined it. Informing his former disciple that only he could stop him, Mū reminded Ōnoki that by taking full advantage of the post-war situation, the dominance of Iwagakure could be assured. Offending the Mizukage, the Raikage and most shockingly Ōnoki with such devious tactics, Mū immediately isolated himself when the battle resumed, pursued only by his student. As the chase concluded after the pair had seemingly clashing several times, Mū declared to an exhausted Ōnoki, that his age had caught up to him and as he disappeared from view, and told him to call Gaara before he died as well. However, telling Mū not to underestimate him just yet, Ōnoki creates several rock clones.
Enter Uzumaki Naruto
As their battle ensues, Mū took Ōnoki by surprise from behind in mid-air, but with Gaara's timely arrival and warning, Ōnoki managed to avoid the blow but threw his back out in the process. As Mū prepared to attack them with his Dust Release, Naruto Uzumaki attempted an surprise attack with his Planetary Rasengan technique, but, sensing the attack, Mū easily evaded it. Believing his technique to be the faster of the two, only the combined efforts of both Gaara and Naruto ensure that the latter lands a hit on a shocked Mū, which sent him hurtling through a nearby rocky outcrop. Colliding with the ground below, Ōnoki swiftly moved in and weighed Mū's body down, which prevented him from moving.
As Mū was being sealed within Gaara's sand, he told Ōnoki that for someone who detested other villages, he worked well with them, to which Ōnoki replied that he had lived long enough to experience such a change. Just as the seal neared its completion, Mū told them "Be careful, I…" but is sealed in full before he could finish his sentence. Some time later, whilst unearthing himself from the rubble left in the wake of Naruto's devastating attack, a damaged, but regenerating Mū, chastised his former disciple for still being naive, having overlooked his body splitting while it was obscured by the prior attack. Unable to remove the seal that bound his other half, Mū was made to await Kabuto's instruction, until the preparations for "that" summoning was finished. After more time passed, Mū performed the Summoning Technique.
Narrowly avoiding the summoned coffin's lid after it was violently blown off, Mū is shocked at the figure who emerged, remarking that the individual behind their reincarnation must have a good understanding of war to summon such a person. Together the pair walked towards Gaara and the Fourth Division, where it is revealed that the one Mū summoned was Madara Uchiha. When Madara noted that all of the shinobi were wearing similarly marked ("shinobi" (忍)) forehead protectors, Mū informed him that an alliance had been formed in response to the on-going war. As Mū was unable to answer Madara's question about who was controlling them, Kabuto overrode Mū's consciousness and used him to talk directly with Madara. As the battle restarted, Mū narrowly avoided Gaara's attack, and attempted to form a Dust Release technique, but it soon dispersed due to his power in his split state.
Mū and Madara vs. The Current Kage
Observing the ensuing conflict, Kabuto continued to speak through Mū's where he remarked upon Madara's demonstration of the Rinnegan and its power, even after the impact of two enormous meteorites. Reforming in the wake of the collision, Kabuto had Mū approach the few remaining survivors of the Alliance, whilst providing Madara with a detailed overview of current affairs. However, when the two resumed their attack, they are repelled by Tsunade and A who had just been transported to the battlefield. Recovering from A's punch, Mū was pursued by Dodai and the Hokage Guard Platoon. As Mei created a thick mist, obscuring the battlefield, Kabuto positioned Mū right in front of Madara but he was easily knocked away by a lightened A.
Once recovered, Mū continued to elude his pursuers under Kabuto's direction, as the battle between Madara and the Kage intensified. Later, with the Impure World Reincarnation being released, Mū's consciousness returns to him but only for a short while as he was enveloped in a light of sorts and began deconstructing, much to his shock, regaining his consciousness only mere moments before starting to break down and his soul returned to the afterlife.