Once upon a time, there was a Braahman called Vishwaanar who was a great devotee of Shiv Jee. Shuchismati was the name of his devout wife. Being pleased by her devotion, Vishwaanar desired to reward his wife. Shuchismatee expressed her desire of having a son just like Shiv. On that account Vishwaanar went to Kaashee to please Shiv by his penance to get a child like Shiv as desired by his wife. He worshipped Vishweshwar Ling there with supreme devotion. Shiv Jee became very pleased by his devotion and appeared before Vishwaanar from his Ling. When Vishwaanar expressed his desire, Shiv agreed to take birth as his son. Vishwsanar came back to his house happily.

In due course of time, Shuchismatee became pregnant and gave birth to a beautiful child. The whole family celebrated the birth of the child. The naming ceremony of that child was attended by all the deities and sages including Shiv and Paarvatee. Brahmaa Jee named the child as Grahapati. When Grahapati attained the age of five, his sacred thread ceremony was performed and within a year he became proficient in all the Ved and other sacred texts.

When Grahapati attained the age of nine, Naarad Jee came there and informed Vishwaanar that demise of Grahapati was imminent because of the evil effects of the planetary combinations. Vishwaanar and Shuchismatee were saddened greatly and started crying. Grahapati then consoled his parents and proceeded towards Kaashee to do penance so that the death could be conquered. Grahapati commenced his penance at Kaashee. The deity Indra arrived there and requested him to demand anything he wished but Grahapati refused him to ask anything from him. Indra became furious and tried to attack him with his weapon - Vajra. Grahapati was very terrified.

Right then Shiv appeared and Indra had to retreat from the scene. Shiv blessed Grahapati by saying - "What to say about this lightning - Vajra, even Kaal Vajra would not be able to kill you." Grahapati became very pleased. The Shiv Ling which he worshipped, later on became famous as Agneeshwar Ling. Shiv made Grahapati the Lord of all the directions.

My Note
The above legend about Grahapati indicate by prayers the effect for told by planets can be removed by good Karm. This is what is told in Astrology as Kaarmik theory. If an astrologer talks about Grahapati, it is an incarnation of Shiv himself and a King above the ministers - Graha.