Here's the briefest account of Sri Niyananda Prabhu's Nativity from Caitanya Bhagavat Adi 2.
Izvara AjnAy Age zrI anantarAma/
rAre avatIrna hailA nityAnanda rAma//
mAgha mAse zuklA trayodazI zubha dine/
padmAvatIgarbhe ekacAkA nAma grAme//
hArAi pandita nAme zudha viprarAja/
mUle pitA mAtA tAne kari pitA vyAja//
kRpAsindhu bhakidAtA prabhu balarAma/
avatIrna hailA dhari nityAnanda nAma//
Upon the command of Izvara, first Sri Ananta Rama manifest his descent in the region of RAra as Nityananda rama.
Upon that most auspicious day, the thirteenth in the bright fortnight in the holy month of Magha, in the village called Ekacaka, he was born from the womb of blessed Padmavati.
The pure and princely vipra named HarAi pandit became the happy father
of him who is the original father and mother of all.
The lord Balarama is an ocean of mercy and bestower of bhakti;
he has manifest his descent bearing the name Nityananda.
mahA jaya jaya dhvani puspabarisana/
saMgope devatAgana karilA takhana//
se dina haite rAramandala sakala/
bArite lAgila punah punah sumangala//
je prabhu patita jana nistAra karite/
avadhuta veza dhari bhramilA jagate//
anantera prakAza hailA hena mate.......................
Great resounding of joyous approbation filled the sky,
and unseen, the celestials sent down a shower of flowers.
From that day the entire region of Rara swelled with prosperity,
and grew ever more auspiscious.
That very lord, who with the intention of uplifting the fallen,
wandered over the land in the guise of an avadhuta.
In this manner has been described the manifestation of Ananta......................