First incarnation in Dashavataras – Matsyavatara
Matsyavatara
According to Hindu puranas, lords Brahma, Vishnu & Shiva are collectively referred as the Holy Trinity (aka Trimurthi). Lord Brahma is the creator and lord Shiva is the destroyer. Lord Vishnu takes care of the dharma and protects the human kind and all the living beings from the hands of evil or demon. Vishnu has descended to Earth in various forms to protect dharma and its followers. The ten such incarnations of lord Vishnu to protect the universe from demons are popularly known as Dashavataras. Each and every incarnation of Vishnu has the same purpose, to protect Dharma. But was done in different means in different avataara.
According to the Vishnu Purana, the story of the first such incarnation of lord Vishnu goes like this :
Matsyavatara was the first incarnation of lord Vishnu. Matsya means fish in saskrit language. Lord Matsya is usually represented as a four-armed creature of which the upper torso is of a man and the lower part is of a fish. Hindu mythology mentions that the great deluge, Jala pralaya, has come upon to cleansify earth when people on earth became very cruel and stopped following dharma. This deluge would have wiped out the entire life form. Hence, Lord Vishnu descended as Matsya Avatar to protect a pious man called Manu and other living creatures that were required for continuation of life on the earth.
Jalapralaya
Once lord Vishnu gave Vedas to lord Brahma and asked him to pass them to people so as to bless them with knowledge. But the people on Earth were in a sad state. They were irreligious and leading disorderly lives without following any dharma. So, Bramha did not want to pass those vedas at that time. Instead, He discussed the situation with Lord Shiva and requested his help to cleansify earth. Thus the decision of Jala Pralaya was taken. Then new life can start on earth to whom knowledge can be passed on. But a demon with a horse-head called Hayagriva ( don’t be confused with Lord Hayagriva, god of wisdom and knowledge) stole the Vedas from Brahma. He hid them deep inside the oceans of the earth.
Lord Vishnu Appearing Before Manu
There was a pious king by name Satyavrata on earth. Satyavrata was popularly known as Manu .His wife was Satya rupa.He was the son of lord Brahma. Manu was praying lord Vishnu for the well-being of mankind. Lord Vishnu decided to protect Manu and other creatures that are required to start a new life on earth after that cleansing.
One day, Manu was praying to the Sun god in a river. He was offering jala tarpanas to sun god. For that he dipped his hands into the river to pick water. That time a small fish came into his palms. It pleaded with him to save his life. He put it in a small vessel. But it soon grew big and hence could no longer fit in that vessel. He then placed it in a tank. Then in a river and then finally in the ocean. But the fish kept outgrowing all those. He then realized that this was not a normal fish.Hence requested the fish to reveal its purpose. The fish then revealed himself as lord Vishnu in Matsya avataara as a four-armed creature with upper torso as a man and the lower as a fish.
Vishnu Protecting Vedas and Life on Earth
Lord Vishnu told Manu that within seven days huge floods would occur and immerse everything on the earth. He said that he will arrange a boat. Manu has to board the boat along with Satyarupa. Also, Manu has to collect a pair of each and every living creature on the earth and place them in the boat.
Meanwhile, lord Matsya, killed Hayagriva and returned the Vedas to Brahma. Manu and Satyarupa boarded the boat with other living things including all animals, medicinal herbs, plants,seeds of trees accompanied by Sapta Rishis. They started their journey according to Lord Vishnu’s guidance. Lord Vishnu came in the Matsya avatara to protect that boat in floods and landed it in a safe new place to start a new life where they could begin human civilization again.