That is a video of Yesudas singing one of the most famous Ayyappa songs – Harivarasanam – in his divine voice. This devotional song is sung every night and is Ayyappa’s lullaby. The Hindu blog has the history of the song.
This divine song which drenches the eyes of Ayyappa devotees in tears was written Kumbakudi Kulathur Iyer. Harivarasanam lyrics were composed in 1950. Kumbakudi Kulathur Iyer used to sing it daily when the temple doors were closed after performing the Athazapuja — serving the last meal of the day to Ayyappa. Today it is known as the Urakku Pattu — or the song that sends Ayyappa to sleep.
In the beginning, the main priest used to play flute while closing the doors of the temple. Harivarasanam became the Urakku Pattu of Ayyappa after the infamous fire incident in the 50s, which burn down the old temple. When the new temple was built and the pujas commenced, Harivarasanam was inducted as the Urakku Pattu — the song to send Ayyappa to sleep.[Who is the author of Harivarasanam?]
The e-Anjali newsletter of Kerala Hindus of North America has more details.
The ashtakam (8 stanza song) was first rendered at Sabarimala in 1955 by Swami Vimochanananda. In those days, only a few ardent devotees managed the difficult pilgrimage to Sabarimala in the deep jungles. The temple remained open during the November to January season but otherwise only on the first day of every Malayalam month. One Sri VR Gopala Menon from Alapuzha used to accompany the Melshanthi (head priest) Thirumeni Eashwaran Namboothiri to the Sannidhanam, and he would often stay there by himself in a shack even when the temple was closed, undisturbed by the wild animals, and often even feeding some animals. He used to sing Harivarasanam as the “urakkupaattu” (lullaby) for ayyappa swami at night. Later, when the Devaswom Board was formed, some say that he was asked to move out and he eventually passed away at a tea estate at Vandipperiyar.
When Thirumeni Eashwaran Namboothiri heard about the passing of the ardent bhaktha, he was deeply saddened. At the end of the day’s rituals, thirumeni was about to close Sannidhanam doors when he remembered the dedication and sacrifice of the bhaktha and he began to recite “Harivarasanam,” starting a tradition that remains unbroken to this day.