As per Hindu literature it's not Saturn which is demon but Rahu and Ketu.

Rahu and Ketu
Two deities are actually demons who managed to gain a place in the pantheon through an act of trickery.

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Their names are derived frm either comets or from the dark and somewhat hostile planets of the solar system (Neptune and Pluto). Depending upon their location in the planetary system and their association with the remaining deities thy are deemed either auspicious or inauspicious

Rahu and Ketu are considered as two strong planets as per the principles of Vedic Astrology. But astronomically, they do not exist. Since Rahu and Ketu are believed to have a strong impact on our lives,

they are denoted as mathematical points while making calculations in Vedic Astrology. When Moon moves from South to North in its orbit and crosses Sun’s path – the incision point is called Rahu or Dragon’s Head.

When Moon transits towards South and crosses the Sun’s path – the incision point is called Ketu or Dragon’s Tail. This point is at 180 degree distance from the previously mentioned incision point (Rahu).

Mythology – The Story of Rahu and Ketu
Rahu was born to Simhika and Viprachitti. Rahu’s mother Simhika (also known as Simhita) was the daughter of Hiranyakashipu (Bhakta Prahlada father). There are several brothers to Rahu and they are Salya, Nabha, Vaataapi, Ilvala and Namuchi.

It is also believed that Rahu is the eldest of the 100 brothers. He also has a sister by name Maahishmati.

There were constant wars between the Devas (gods) and the Asuras (demons). The Asuras always won, due to their guru Shukracharya Mritha Sanjivini power, which the guru had obtained as a fruit of his penance, propitiating Lord Shiva.

The power of this Mritha Sanjivini is, it can resurrect the dead, back to life. In this way, each Asura who was killed in the war by the Devas, was brought back to life by this mantra. Thus, this ensured constant defeat for the Devas and the Asuras continued to rule the heavens.

It so happened that Shukracharya (Venus) was blessed with the knowledge of “Sanjivini Mantra” also known as the Maha Mrityunjay Mantra after great Tapasya for the benefit of all people (universal benefit) Armed with this knowledge Shukracharya could now revive all the dead people

Shukracharya would revive all the dead Asura soldiers while those on the other hand the Devas were getting killed. Eventually, being hugely outnumbered, Indra lost his kingdom. When the defeated Devas approached Lord Vishnu for his protection,

he advised them to perform “Samudra Manthan” to attain Amrit (Nectar) consuming which would make them immortal

Also, Lord Indra, the king of the Devas, went to the abode of Lord Vishnu, with all the Devas, and, prayed to the Lord, asking him for a solution.

Lord Vishnu, asked the Devas to churn the milk ocean (Sagar Manthan), and, consume the amrit that emerges out of this ocean. For this, they would need to take the help of the Asuras.

Lord Indra, went to heavens, and, sought for peace, and, said that henceforth,

the Devas and Asuras should never wage war against each other, and, told them the need for churning the ocean of milk and the power of consuming the Amrit. The Asuras, agreed, and inside their evil minds, they thought,

once the churning is over and the Amrit is obtained, we will steal the Amrit. Thus, the great churning the ocean of milk began, with Mount Meru as the churner and the King of Serpents, the Lord Vishnu’s bed, the Great Naga, Vaasuki, was requested to be used as a rope.

From, advising the Good Devas to churn the ocean, the Divine Lord Vishnu, sided the Devas and protected them on time from every danger, proving that God is always with the Good, and he does not destroy evil, but, controls the evil,

so that, both Good and the Evil, co-exist, and one can learn to distinguish the Good from the Evil. Lord Vishnu, had told the Devas to remain by the tail side of Vaasuki, so as to avoid the poison emerging out of Vaasuki’s mouth, and, the Asuras,

stood by the head side of the Vaasuki, not knowing the poisonous power of the Great Vaasuki. As the great churning was taking place, the huge Mount Meru, began to slip through the Vaasuki ropes and slide into the ocean, Lord Vishnu, took the form of a Tortoise (Korma Avatar)

and went down the ocean bed and lifted up the Mount Meru and balanced it.

After churning the ocean of milk (Kshir sagar) for several years, first came the Halahala (poison), this poison can destroy all the creations, so, without even thinking about himself, Lord Shiva,

saw this poison emerging out of the ocean and immediately began drinking it, Goddess Parvati saw this extreme compassion of her divine lord for the Good beings of the world, and, blocked his neck, by pressing it, so, the poison did not enter Lord Shiva’s stomach,

instead stayed in his throat, and, his neck began turning blue, thus, he came to be known as Neelakantha, the one with a blue throat.

The results of churning ocean, gave rise to various things that came out of the ocean: Surabhi, the Kamadhenu or divine cow, apsaras (angels),

divine horse (Uchhaishravas), divine elephant (Airavata), Varuni (half-angel half-demoness), Kaustubha (divine necklace), Parijata (divine blossom tree with flowers), kalpaka vrikshah (another divine tree), Shankh (Vishnu’s conch), Tulsi plant (Basil) etc.

Finally, Goddess Lakshmi emerged out of the ocean, and, the divine Lord Vishnu got enticed by her beauty and offered to accept her as his consort to his divine abode Vaikunthah, Goddess Lakshmi accepted Vishnu as her Lord.