CC Ādi 5.116
sei prabhu nityānanda — sarva-avataṁsa
That Lord Viṣṇu is but a part of a part of a plenary portion of Lord Nityānanda, who is the source of all incarnations.
The Lord of Śvetadvīpa has immense potency for creation and destruction. Śrī Nityānanda Prabhu, being Baladeva Himself, the original form of Saṅkarṣaṇa, is the original form of the Lord of Śvetadvīpa.