by Swami Sivananda
After reaching the deity identified with lightning, the soul reaches the world of Varuna.
Tadito’dhi varunah sambandhat IV.3.3 (520)
After (reaching) the deity of lightning (the soul reaches) Varuna, on account of the connection (between the two).
Taditah adhi: after the deity of lightning; Varunah: (comes) Varuna (rain god); Sambandhat: on account of connection.
The enumeration of the stations of the journey is continued.
In the Chhandogya text we find, "From the sun to the moon, from moon to lightning." In the Kaushitaki Upanishad we find, "From Vayu (wind) to Varuna." Combining the two texts we have to place Varuna after lightning, on account of the connection between the two (lightning and Varuna). The broad lightnings dance forth from the womb of the clouds with the sound of deep thunder and then water falls down. "It lightens, it thunders, it will rain" (Chh. Up. VII.11.1). Varuna is the god of rain and lightning precedes rain. So after lightning comes Varuna.
After Varuna come Indra and Prajapati for there is no other place for them. The Kaushitaki text also puts them there.
The complete enumeration of the stages of the path of the gods is as follows: first the deity of fire, then the deity of the day, the deity of the bright half of the month, the deities of the six months when the sun travels to the north, the deity of the year, the deity of the world of gods, the deity of the air, the sun, the moon, the deity of lightning, the world of Varuna, the world of Indra, the world of Prajapati, and finally Brahmaloka.