As I discuss in this answer, one of the most prominent incarnations of Vishnu in ancient times was the sage Narayana, twin brother of the sage Nara and son of Yama god of death. Nara and Narayana were famous for engaging in Tapasya (deep meditation) in Badrikashrama. When Indra tried to distract them with his Apsaras (nymphs), they created an Apsara even more beautiful, Urvashi. And as I discuss in this answer, when the king Dambhodbhava challenged them to battle they were easily able to humble him. As I discuss in this question, Nara and Narayana were famous enough to have the opening verse of the Mahabharata dedicated to them. And they were reborn as two central characters in the Mahabharata, Arjuna and Krishna.
But my question is about something else that the sage Narayana is known for, described in the Shatapatha Brahmana of the Yajur Veda:
Purusha Nârâyana desired, 'Would that I overpassed all beings! would that I alone were everything here (this universe)!' He beheld this five-days’ sacrificial performance, the Purushamedha, and took it, and performed offering therewith; and having performed offering therewith, he overpassed all beings, and became everything here. And, verily, he who, knowing this, performs the Purushamedha, or who even knows this, overpasses all beings, and becomes everything here.
For those who don't know, the Purushamedha Yagna is a Vedic Yagna (fire-ritual) where you pretend like you're preparing to do a human sacrifice, but then at the last minute you stop yourself from actually carrying it out. Now since it took five night for Narayana to complete the Yagna, the ancient sect that worshipped Narayana came to be called the Pancharatra (five-night) sect; this sect was influential in the development of a Vaishnavism, with the sacred texts of the Pancharatra sect later becoming important Vaishnava scriptures.
My question is, what exactly was the result of Narayana's five-day Yagna that was so important that an entire sect of Hinduism was started around it? What does it mean to say that "he overpassed all beings, and became everything here"? Did he die and turn back into Vishnu when the Yagna was over?
By the way, Narayana is the sage who heard the Purusha Sukta of the Rig Veda from the gods. (See Book 10 Hymn 90 in the Rig Veda Anukramani in my answer here.) The Purusha Sukta is a hymn dedicated to Purusha, another form of Vishnu, and the hymn plays an important role in the Purushamedha Yagna; see verse 12 in the next chapter of the Shatapatha Brahmana. Could his hearing the Purusha Sukta have some connection to his performance of the Purushamedha Yagna and its aftermath?