Hiruzen is the son of Sasuke Sarutobi and was trained by the 2nd Hokage with his teammates being Koharu and Homura, but he also partically trained by First Hokage as well. Two brothers and past hokages has big belief and intereset on young sarutobi and Hiruzen has big respect on them. He'd then later go on to train Jiraiya, Tsunade, and Orochimaru. He was chosen over his friend and rival Danzō to be the 3rd Hokage. Danzō created "Root," a secret group within the Leaf village that swore to do what the peace-loving Hiruzen could not, but Hiruzen found out and had the group disbanded.
Hiruzen once hoped Orochimaru would succeed him as Hokage, but he saw the darkness inside him and started to favor Jiraiya. With Jiraiya on his journey and the threat of Orochimaru's corruption, Hiruzen would select Minato to succeed him as Hokage. Also Jiraiya don't want to be a Hokage, because he can't stopped his friend(Orochimaru) when he leaves the village.
However, on the night of the Kyūbi's attack, Hiruzen organized everything for Naruto to be born and his wife Biwako was tasked to receive Naruto when he was born, however the events that led to the Kyūbi's rampage resulted in Biwako's death, and with Minato off dealing with an unknown threat, Hiruzen ordered the civilians to be protected while he prepared himself to fight the Kyūbi. Hiruzen and the other ninjas drove the Kyūbi out of the village, out of fury it prepared to shoot a tailed beast bomb but Minato appeared and teleported the Kyūbi to a far off location. Hiruzen quickly followed after to help, but was stopped by Kushina's barrier upon his arrival. He was forced to witness Minato and Kushina die as they sealed away the Kyūbi. Hiruzen would then once again resume his position as Hokage since Jiraiya wasn't available. Also Jiraiya has big regret on that situation because he is blame himself about death of his favorite student (4th Hokage).
After the 4th Hokage's death, the Uchihas would be blamed for the Kyūbi's attack and, despite Hiruzen's protests, the Uchihas were segregated and forced to live in a corner of the village by the elders. Fugaku, the leader of the Uchihas, planned to take over the village in retribution. Hiruzen and the elders caught wind of this and Hiruzen began negotiations, which didn't work. The elders feared that if the Uchihas attacked that it would leave the village vulnerable and it was necessary to kill the Uchihas before a 4th ninja war occurred. To add to Hiruzen's grief, he would later discover the horrible crimes that Orochimaru committed. But despite his anger, he couldn't bring himself to kill his student, allowing Orochimaru to flee the village.
When Naruto failed to graduate the academy, he was tricked by Mizuki to steal a scroll containing forbidden jutsu. Hiruzen was the first to see Naruto, but being a closet pervert, he was stunned by Naruto's "Sexy" jutsu which allowed Naruto to escape. Hiruzen sent out a squad to find Naruto and he watched with his crystal ball as Naruto was proven innocent and defeated Mizuki to protect his friend and sensei, Iruka. After Naruto became a Genin, Hiruzen watched first hand as Konohamaru, his grandson, and Naruto become friends and worried that Naruto would be a bad influence, but he also watched as Naruto helped change Konohamaru and make him a better person. Hiruzen talked to Kakashi and told him about his new student. Then after Naruto was placed on in Team 7 with Kakashi, Sakura, and Sasuke, Hiruzen gave into Naruto's begging and gave him a higher ranking mission, however the client lied and it turned out to be a much more dangerous mission; but Team 7 prevailed.
Eventually it will be the Leaf village's turn to host the Chūnin exams. the 4th Kazekage, leader of the Sand village, decided to attend and watched up high next to Hiruzen. What Hiruzen and the rest of the Leaf village didn't know is that the Chūnin exams was a cover for a united invasion by the Sand and Sound villages, the latter being Orochimaru's brain child. However what the Sand village didn't know is that Orochimaru impersonated their Kazekage... after killing the real one. Orochimaru brought Hiruzen to a large rooftop where the Sound 4 were waiting and constructed a barrier to stop them from leaving, or anyone to stop the fight. Hiruzen would have to fight his former student to the death.
The fight started with Hiruzen using Roof Tile Shuriken, Orochimaru evaded and used Hidden Shadow Snakes to bite Hiruzen's neck, but it was an Earth Clone and Hiruzen used Earth Flow River to sweep Orochimaru away and stopped Orochimaru from using a jutsu by attacking him with a combination of Earth Dragon Missiles and Dragon Flame. Orochimaru managed to escape harm by using Leech All Creation to fuse with the roof and move away. Hiruzen then attacked with Shuriken Duplication, but Orochimaru used Resurrection to the Impure World to conjure coffins to shield him. Hiuzen realized what was going on and used Technique Neutralizer to stop the summoning of the third coffin. Out of the coffins came the The First Hokage and The Second Hokage, Hiruzen's teachers, as Orochimaru's mindless undead servants.
Things were bad and, being almost 70, Hiruzen knew that he wasn't as powerful as he used to be. Hashirama and Tobirama charged at Hiruzen, attacking separately, but Hiruzen was able to force them back. He then used Fire Dragon Napalm but Tobirama shielded himself, Hashirama, and Orochimaru with Water Encampment Wall; Tobirama then went on the offensive with Water Collision Destruction, at which point Hiruzen used Earth Style Wall to protect himself. Hashirama raced in to close the gap, Hiruzen spat fireballs at him, but Hashirama deflected them. Hashirama then bested Hiruzen in taijutsu, at which point Tobirama used water to wrap around Hiruzen and drag him underwater to drown him, but Hiruzen escaped and climbed to the top of the rock wall he had created. Tobirama responded by shooting Water Jets to cut the wall down.
Hashirama then used Birth of Dense Woodland to chase Hiruzen and bind him, but Hiruzen was able to summon Enma the Monkey King. Enma forced Hashirama and Tobirama away long enough to free Hiruzen by transforming into a large size-altering rod and breaking the trees. Hiruzen then took hold of Enma in his rod form and charged at Orochimaru, whom had brought out his Kusanagi sword. The two squared off, but their fight was interrupted by Hashirama and Tobirama, and Orochimaru managed to take Hiruzen down. Orochimaru walked up to the fallen Hiruzen, but Hiruzen brought Orochimaru to his knees and Enma reached out and tried to snap Orochimaru's neck, but it was an Earth Clone. Orochimaru showed himself and Hiruzen kicked him away, then revealed that he had placed Paper Bombs on Hashirama and Tobirama and blew them up, but they pulled themselves back together.
The fight stopped for a moment while Orochimaru revealed that he had gained an immortality jutsu by taking the bodies of others. Hashirama used Birth of Dense Woodland again, this time Enma protected Hiruzen by turning himself into a cage. Hiruzen knew the only way to win was to use Minato's technique to pull out the souls of Hashirama and Tobirama. He created some Shadow Clones to fight for him while he used the Reaper Death Seal.
Hashirama used Bringer of Darkness to envelop everything in darkness and attack Hiruzen, but the Shinigami reached out through the Shadow Clones' chests and pull the souls from Hashirama's and Tobirama's bodies, revealing that the sacrifices used to summon them were Zaku and Kin. Orochimaru and Hiruzen began fighting again, Hiruzen knocked away the Kusanagi, but Orochimaru used Hidden Shadow Snakes to pull Enma from Hiruzen. Hiruzen got in close and grabbed Orochimaru to pull out his soul, however Orochimaru showed that he could control the Kusanagi even when it's not in his grasp and it stabbed Hiruzen despite Enma's effort to stop it.
Orochimaru was surprised that Hiruzen didn't try to avoid it, whom revealed that he's going to die anyway. Hiruzen wasn't able to steal Orochimaru's soul, but he did manage to take away the use of Orochimaru's arms. In his death he forced Orochimaru to stop the invasion, and took what Orochimaru valued most.
Later on, Orochimaru, curious about Sasuke and his new motivations, decided to aid him in the ensuing war and resurrected Hiruzen and the other Hokage upon getting his arms back. They arrive into the fray in time to face off against Madara, Tobi, Zetsus, and the Jūbi.
Hiruzen has displayed many jutsus: "Technique Neutralizer" where he he cancels his opponent's technique. "Groundhog Decapitation" where he travels underground and pulls his opponent down to their necks. "Crystal Ball" where he is able to spy on events remotely." Because, he has sensor type chakra. He also use this ability against Hashirama's "Bringer of Darkness Technique" then he found his former masters in the darkness. Earth Clone" where he creates a duplicate of himself out of the ground that will disperse after sustaining damage. "Shadow Clone" where he creates copies of himself that are capable of fighting and using techniques, but can't take very much damage without dispersing. In addition to those, Hiruzen is outright capable of using all five elements.
"Shuriken Duplication" where he throws a single shuriken and then creates many physical copies of that single shuriken. " Roof Tile Shurikens" where he levitates and flings tiles off of rooftops at his opponent. "Moving Land River" where causes the ground beneath his opponent to move rapidly. "Mudslide Wall" where he spits a gigantic protective rock wall. "Earth Dragon Missiles" where he creates what looks like a dragon's head to come out of the ground and shoot compressed earth at his opponents. "Dragon Napalm" where he projects a continuous 3-pronged stream of fire from his mouth. "Flame Shot" where he spits balls of fire. And "Reaper Death Seal," a technique that he learned from Minato which pulls the soul of his opponent out of their body, at the cost of the user's own life.
He's also capable of summoning monkeys, most notably Enma the Monkey King. Enma is capable of using the technique "Adamantine Staff" where he transforms into a near-indestructible, size altering pole which Hiruzen uses as a weapon. Even while in the staff form, Enma is still able to see and extend himself or just parts of himself to attack the opponent. He can also use "Indestructible Prison Wall" where he creates copies of himself while in his pole form and becomes an interlocking cage. Enma's adamantine form is strong and tough enough to cut vines of wood release.
Hiruzen is the only one who is capable of use and mastered all five main element releases without using ying and yang releases. He is the true Professor of the Shinobi World.