Black Books- No. 1
According to Sonu Shamdasani (the translator of Jung's works), the serpent imagery in the Black Books (2020) refers to Kuṇḍalinī Yoga. In Black Book No. 1, Jung refers to Kuṇḍalinī Yoga in his mandala drawings, describing this sequence in The Secret of the Golden Flower (2014).
"This symbolism refers to a quasi-alchemical process of refining and ennobling. Darkness gives birth to light; out of the 'lead of the water region' grows the noble gold: what is unconscious becomes conscious in the form of a living process of growth. (Indian Kundalini Yoga offers a perfect analogy). In this way, the union of consciousness and life takes place."- Page 63. And noted in Alchemical Studies (1983), Page 24/Commentary 34.
Jung discusses the union between the conscious and unconscious, the dark night of the soul, rebirth, all of which are symbolic layers included in the process of Kuṇḍalinī. From the serpent imagery, we see many linguistic hints connecting to "birth", "rising up", "the dragon", "divinity", "evil serpent", and "the spirit world".
We see more dragons appear later on in Jung's Black Book (page 143 and 145), which is also relatable to Middle Eastern iconography of the serpent, particularly the "lion-bird"; a species of dragon that was sacred to Sumerian god Ningishzida (the deity of vegetation and the underworld).- see Entwined Serpents essay by E. Douglas Van Buren, Page 55.
Black Book, Page 136-137