Sati and Uma
Although Shiva is worshipped as the ruler of the Three Worlds, his power, or Sahkti, is represented by his wife, the Devi (Goddess).
Shiva married Sati against the wishes of her father, Daksha. When Daksha performed a ritual sacrifice invited all the Deva, but not Shiva. Sati was hurt by the insult to her husband, and when she questioned her father, received only harsh words. Sati, devastated, fixed her mind on her Lord, and invoking the Tapas' flames immolated her body in them.
Sati, in her love and devotion to Shiva, took rebirth as Parvati to be with Him again.
In other tales it is said that Uma Haimavati (uma = light), looking at Shiva in despair for Sati's death, took pity on His endless grief and revealed herself as Parvati, to comfort Him.
Uma - The Golden Goddess
Once a rishi was imparting to his student's spiritual knowledge. When a lesson was over, the students were free to ask questions on the day’s lesson. One day the students asked: "Sir, who makes the mind desire? Who makes the eyes to see, the ears hear and the tongue speak?"
The rishi said: "The One Great God causes all these things. He is everywhere. He is the source of all things. He who understands this truth becomes immortal." But the students said they did not understand him. So the rishi told them a story.
Once the Devas won a battle against the Asuras. The Devas, however, did not realise that it was the Great God who made them win. The Devas thought they won the war by their own strength and in their ignorance they became proud of what they had achieved. The Supreme God or Brahman wanted to cure them of their pride, so he suddenly appeared before them. The Devas could not say who it was, all that they could know was that it was a wonderful Being, so they decided to find out who it was.
First they sent Agni, the God of fire, to find out whom it was. When Agni went to the Strange Being, the latter asked, "Who are you?" Agni replied: "I am Agni, the powerful. I can burn anything in this or other worlds. I know everything!" The Supreme Being said: "Oh! Is that the fact? Look, here is a blade of grass. Burn it, if you can!" "Pooh!" said Agni and rushed upon the blade of grass. Try however much he did, he could not even touch the edge of the grass. Ashamed of his failure, he returned to the Devas and admitted his defeat.
The Devas next sent Vayu, the wind good, to find out whom that Strange Being was. "Who are you?" asked the Being. "I am Vayu, the great. I can sweep away anything in this world with my power." "Can you really," said the Being. "Please sweep away this blade of grass. Vayu rushed at it, but the straw did not move an inch. Vayu too returned to the Devas and reported his failure.
Then the Devas sent Indra, their King, to the Strange Being. When Indra went, the Strange Being was no longer to be seen, but in its place stood a charming woman. She was Uma, the Goddess of spiritual knowledge. Indra asked Uma: "Great Lady! Who was that Strange Being that was here!" Uma replied: "Fool! It was Brahman, the Supreme Being. It was he that won the battle against the Asuras for you. Not knowing it, you bragged amongst yourselves. He came here to teach you how worthless your powers are without His grace, and to cure you of your pride!"
On hearing this, Indra went back and reported this matter to the Devas. Then the Devas realised their mistake and begged to be forgiven. Later, they learnt the sacred knowledge about God, in humility.
The rishi said finally: "This knowledge comes in a flash. We must pursue and develop it further. That is our object in life; for, God is Truth. All knowledge is His limbs. Penance and doing good to others are the means of furthering that knowledge."
This story occurs in the great Upanishad called Kena.