What scriptures describe Brahma's son Sanatkumara being reborn as Shiva's son Kartikeya?

Question

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

32. 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.

Answer 1

The 37th Chapter of the Mahatmya Kanda of the Tripura Rahasya details the story of Sanatkumara incarnating as Kartikeya.

Lord Dattatreya relates the story of Lord Muruga to Parashurama, listening to which Parashurama asks Dattatreya:

Kathamesha Mahabhagah Skandah Tam Asuresvaram|
Jitavaan Shivavishnvendra-Durjeyamapi Samyuge|| (37.78)

"O Lord, how was this greatly fortunate Skanda able to conquer the king of Asuras , who was undefeatable by even Shiva, Vishnu and Indra in battle?"

To which Dattatreya replied that Lord Sanat-kumara, the son of Brahma was ever peaceful and content with himself. He dwelt on the Mountain named Rshabha. Once, Lord Brahma arrived to meet his son, and Sanatkumara related a dream he had seen, of a great battle between the devas and asuras.

Yuddhamaseet Mahabhimam Asuranam Tatha Amaraih|
Tatra Sarve Maya Yuddhe Nihataah Balavattaraah|| (37.85)
Asuraas Tat Kathamidam Nirhetukam Abhut Vada|

"O Father, I saw a dream, of a great war between the asuras and the devas. I saw myself slaying all the formidable asuras in the war. I want to know how such a baseless thing could happen!"

Lord Brahma replied that in a past life, Sanatkumara had been a Brahmana, who having heard the account of the battle between Devas and Asuras, had vowed to defeat all the asuras and return prosperity to the devas, with the fire as his witness. But he attained his death before he could fulfil his vow, so instead of getting Moksha, he incarnated as Brahma's son Sanatkumara. Brahma also told him that whatever he had seen in the dream would actually take place.

When Sanatkumara asked Brahma how this was possible, Brahma told him that Sanatkumara himself would decide how this would take place, and returned to Brahmaloka.

As Sanatkumara meditated on this statement, Lord Shiva and Devi Parvati passed by his hermitage. Seeing his composed face, Parvati (who had not seen Sanatkumara before as she was newly married) asked Lord Shiva who he was. When Shiva told her about Sanatkumara, she expressed her wish to meet him.

When both Mahadeva and Girija reached the hermitage, Sanatkumara didn't notice them. After waiting for a long time, Mahadeva decided to test Sanatkumara and flew into a rage. He scolded Sanatkumara saying that his conduct did not befit him, the son of Brahma. When Sanatkumara 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. Sanatkumara laughed and said,

"Saadhayasva Mahadeva Esha Me Kaankshito Varah|| (37.118)
Maayavinam Maheshaana Naaham Vepayitum Kshamah|
Mahavaatair-Ivaakasham Bruhi Te Yadi Vaanchitam||(37.119)

"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,

Yadi Dasyasi Me Kamam Prateeccha Mama Putrataam|| (37.120)
"If you shall grant my wish, then become my son!"

Sanatkumara smiled and said, "I shall become your son, but only yours, because Parvati Devi hasn't asked this boon of me!"

Parvati immediately argued that since the wife was entitled to half of her husband's fortunes and misfortunes, she also was entitled to being his mother. Sanatkumara consented and said, "When Bhasmasura had sought the boon to turn anyone to ashes, and tired it on Shiva, he had disappeared, hearing about which you had become aggrieved and turned into a forest of reeds. I shall incarnate into that body of yours; hence you will be my mother."

So saying, Lord Sanatkumara granted them leave, and was born as their son, in the lake (Poigai in Tamil), in the forest of reeds, Sharavanam. Thus he became Lord Muruga.