As I discuss in this question, by far the most popular school of Hindu philosophy is the Vedanta school, which bases its tenets on the doctrines laid out in the Brahma Sutras, a work by the sage Vyasa that summarizes and systematizes the philosophical teachings of the Upanishads. You can read the Brahma Sutras here. In any case, Adhyaya 3 Pada 3 Sutra 32 of the Brahma Sutras says this:
Topic-19: People with a Mission
- Those who have a mission to fulfill continue in the corporeal state as long as the mission demands it.
All the commentators interpret this Sutra as saying that although in most cases when a person attains knowledge of Brahman they attain Moksha as soon as they die, in a small number of cases they take another birth in order to fulfill some role that is vital to sustaining the Universe. In Adi Shankaracharya's commentary on this Sutra, he gives some example of people who were born again for a purpose even after attaining knowledge of Brahman:
The question here is whether for him who has reached true knowledge a new body originates after he has parted with the old one or not.--But an objection is here raised at the outset there is really no occasion for inquiring whether knowledge when reaching its perfection brings about its due effect, viz. complete isolation of the Self from all bodies or not; not any more than there is room for an inquiry whether there is cooked rice or not, after the process of cooking has reached its due termination; or, for an inquiry whether a man is satisfied by eating or not. Not so, we reply. There is indeed room for the inquiry proposed, as we know from itihâsa and purâna that some persons although knowing Brahman yet obtained new bodies. Tradition informs us, e.g. that Apântaratamas, an ancient rishi, and teacher of the Vedas, was, by the order of Vishnu, born on this earth as Krishna Dvaipâyana at the time when the Dvâparayuga was succeeded by the Kaliyuga. Similarly, Vasishtha, the son of Brahman's mind, having parted from his former body in consequence of the curse of Nimi, was, on the order of Brahman, again procreated by Mitra and Varuna. Smriti further relates that Bhrigu and other sons of Brahman's mind were again born at the sacrifice of Varuna. Sanatkumâra also, who likewise was a son of Brahman's mind, was, in consequence of a boon being granted to Rudra, born again as Skanda. And there are similar tales about Daksha, Nârada, and others having, for various reasons, assumed new bodies.
In my answer here I discuss Apantaratamas' rebirth as Vyasa, and in my question here I discuss Vasishta's rebirth as the son of Mitra and Varuna. But my question is, what scriptures describe Brahma's son Sanatkumara being reborn as Shiva's son Kartikeya, aka Skanda or Muruga?
For those who don't know, Sanatkumara is one of the four Kumaras, the first four mind-born sons of Brahma who are most famous for cursing Jaya and Vijaya as I discuss here. In any case, the only reference I know of to a connection between Sanatkumara and Kartikeya is in the Chandogya Upanishad, at the end of a dialogue between Sanatkumara and the sage Narada:
The venerable Sanatkumâra showed to Nârada after his faults had been rubbed out, the other side of darkness. They call Sanatkumâra Skanda, yea, Skanda they call him.
But are there any scriptures which describe the story of how and why Sanatkumara was reborn as Kartikeya? I'm particularly interested in Adi Shankaracharya's statement that it was done "in consequence of a boon being granted to Rudra". What was the boon, and who gave it to Shiva?
By the way, it's interesting to note that this chapter of the Srimad Bhagavatam says that the four Kumaras are incarnations of Vishnu. So that would mean that Kartikeya is an incarnation of Vishnu! That would certainly put the discredited theories that the Venkateshwara statue is Kartikeya in a whole new light.
Also, this chapter of the Adi Parvati of the Mahabharata says that Krishna's son Pradyumna is an incarnation of Sanatkumara, which would put Pradyumna's fight with Kartikeya, which is part of the fight between Krishna and Shiva that I discuss here, in an interesting light.