lord ayyappa, the son of hari and hara. was born to fulfill a deep purpose of killing mahishi, the sister of mahishasura, the demon king killed by devi karthyayini, mata durga.
mahishi had this boon of not being killed by anyone accept the son of hari and hara. so, hari took on the form of mohini and united with hara, it is said, and through their maya, was born the son destined to kill mahishi.
brought up by the king of panthalam, after he had his purpose complete, sat to medtate and shover his blessings to the devotees in a place known as shabarimala. the ayyappaashtakam was sung is sung to make the hariharathmaja to sleep, a lullabai for the dear son of hari, aka mohini and hara, aka shiva.
know the lyrics of the same ashtakam here,, in english with its meanings.