Hagoromo was a monk, he was the son of princess Kaguya Ootsutsuki whom ate the Shinju's forbidden fruit and gained the power to stop armies, a power that he and his brother Hamura would inherit when she conceived them. The God Tree, Shinju, was angry however and came to take back it's power, so Hagoromo and his brother used their power to stop the Shinju by sealing it in Hagoromo. People then worshiped him as a god and he created his own religion, Ninshuu, and taught ninjutsu to people in order to better their lives and wished to bring peace to the world. He also had two sons, the elder Indra inherited Hagoromo's spiritual energy and the power of his eyes, while his younger son Ashura inherited his physical energy and the power of his body.
Sadly he began to die before he could achieve his dream and he decided to entrust one of his sons to continue his dreams. Indra believed that peace could only be attained through might, while Ashura believed peace could be achieved through love, and with that Hagoromo chose Ashura to carry on his legacy and lead the people. Then finally, realizing that his death would release the Shinju back into the world, he divided it's chakra into nine pieces, forming the Bijuu, and he created the moon which trapped Shinju's lifeless husk inside of it.
However, after his death people turned ninjutsu into a weapon and used it to wage war against each other. On top of that, his older son attacked his younger son out of jealousy, believing that he was Hagoromo's rightful heir. This formed a rivalry between them and their descendants, the Uchiha and the Senju, that spanned centuries. Ultimately, Hagoromo and his accomplishments were eventually considered fairy-tales.
Powers and Abilities
Hagoromo was the first to discover ninjutsu and taught it's usage to others, the first he was known to have created was a fuuinjutsu to seal Shinju within himself to imprison it and subsequently gained it's powers. This is the same seal that's used currently to seal the Bijuus.
He also possesses the Rinnegan that lets him see chakra and gives him access to all 6 chakra types: Katon, Fuuton, Raiton, Doton, Suiton, Onmyoton. In addition, it's most notable abilities are it's gravity manipulation, body modification, telepathy, summoning, chakra absorption, and resurrection; each based on one of the 6 paths of Buddhist reincarnation in which he derives his title from.
He also possessed Banbutsu Sozo, a jutsu that allowed him to use his spiritual energy to make forms and then use physical energy to give those forms substance; literally turning fantasy into reality. He used this jutsu for many things, including creating the Bijuu from Shinju's chakra.
In addition, Hagoromo was said to possess five sacred treasures which would come into the possession of two of his suspected decedents, the Gold and Silver Bros. Three of the items work together to seal people: the "Golden Canopy Rope" paralyzes and extracts the soul upon contact, the "Seven Star Sword" cuts the soul free and curses it, and upon activating the curse the "Crimson Gourd" absorbs the person whole; the curse being not to say the word you use the most, although you'll also be sealed if you remain silent for too long or say words that sound like your most used word. Another item is the "Banana Palm Fan," capable of releasing any of the five elements with a swing, and finally there's the "Amber Purifying Pot" that's capable of sealing someone by getting them to respond to you.
The drawback to these items is that they require a large amount of chakra to use, and prolonged use would kill an average person.