Who is Lord Sanat Kumara?

According to publications of Theosophy, Lord Sanat Kumara is an "Advanced Being" at the Ninth level of initiation, who is regarded as the 'Lord' or 'Regent' of Earth (The Head of The Spiritual Hierarchy for Planet Earth) and of the humanity, and is thought to dwell in Shamballah.

Scriptures, especially the Mahabharata, say that it is Sanath Kumara who comes down to help the Divine Plan of Creation, when Lord Krishna came to be. He was born to Lord Krishna as his son, and he was named Pradyumna. Sanath Kumara incarnates himself through the Divine Mother Rukmini as Pradyumna. On completion of his work, He again gets back to his original position and takes to the seat in Shambala as Sanath Kumara. Sanat Kumara is also documented as one of the four Kumaras, the first four mind-born sons of Brahma who are most famous for cursing Jaya and Vijaya.

Sanat Kumar was reborn as Lord Shiva’s son, Kartikya as part of a test that Lord Shiva had conducted. When both Mahadeva and Parvati Devi reached his hermitage, Sanat Kumara didn't notice them as he was meditating. After waiting for a long time, Mahadeva decided to test Sanat Kumara and flew into a rage. He scolded Sanat Kumara saying that his conduct did not befit him, the son of Brahma. When Sanat Kumara was addressed thus, he opened his eyes and said, "I won't be frightened by your curses, nor will I be excited by your boons. I have conquered both fear and excitement, thus you cannot make me react in any way, either ordinary or extra-ordinary." When he had replied thus, Mahadeva was pleased and asked him to seek a boon. Sanat Kumara laughed and said, "O Great Lord, Mahadeva, I cannot be agitated by illusions or illusionists, just like hurricanes cannot disturb the sky. Therefore, I seek this boon that, if you have any wish, you may seek it from me. Upon hearing this, Mahadeva said 'Om' and continued, "If you shall grant my wish, then become my son!" Sanat Kumara smiled and said, "I shall become your son”.