Besides the Moon XVIII, the High Priestess is one of the most Neptunian Major Arcanas. She is the pandan to the male archetype of the Magician - she is the sorceress and the witch of Oz. She is the current of the subconsciousness and she is the keeper of the secrets of Life, as well as Death. She is the flow of the Lethe, and she is the sustenance Souls drink in order to forget their previous lives and prepare for the next one.
The honed diadem on her head is the connection to Rowena Ravenclaw's diadem (the founder of the Ravenclaw Hogwarts house) which is said to have contained of her otherworldly wisdom and wit. She is dressed in light-blue garbs and she is seated between the two pillars - one white and the other black, pillars that represent the gates between Life and Death, and she is its keeper.
She is both Maat and Anubis, and she watches the sum of Soul's lessons during life.
When it comes to divination, the High Priestess stands for a secret that is yet to be revealed, inner illumination, enlightenment, wisdom, a sacred bond between the lower self and Higher Self, between the mind and the heart, and combined with Lovers, the 2 of Cups (or the Devils) it stands for a twin flame connection.
Perthro Rune (Elder Futhark) is a parallel for the High Priestess when it comes to divination, and you can expect to have or make some kind of a revelation in the day to come after the High Priestess pops up in a reading.
When she is combined with the Empress, she can represent an auspicious birth and the birth of an augur. When it comes up in a reading combined with the Judgement, or the World, it can foretell the event of the soul-passenger (when the soul from the ether enters the already occupied body by the soul, and it changes the original blueprint of the destiny of the said body).
Since the High Priestess is not one of the more mundane arcanas, she is often overlooked or not researched/observed enough for commoners to fully grasp and comprehend the full extent of her powers and divinatory meanings. This is why it's very important to lead a meticulous BoS or a spiritual journal and be as detailed as possible when the HP shows up in a reading. The initiates who follow her are often full of Pisces placements or are Neptunian doms, as they are natural gatekeepers and watchers of the borders between Life and Death.
That may be so, but HP should be the star spirit for natives who actually lack Neptunian energies and Pisces placements, especially those who lack the Water element in their chart, as the HP has the audacity and potency to compensate for the lack of Water lessons and teachings in a chart.
The High Priestess spirit should not be overused, however, especially by natives who naturally have strong Water placements and/or are Neptune dominants, as HP can help them get even more ungrounded and untethered to the 3D reality, and that is a lack of balance, which is not the point of any Fool's journey.
So beware of how you approach her, and how often and to be careful not to lose track of when you're being under her influence.
She is a great spirit to ask for help in connecting with your Disir, and if you're adopted or have never met your biological mother, HP can aid you in discovering the secrets hidden from you that lie (dormant) in your DNA.
HP is also a perfect aid in the opening of the 3rd Eye, especially if you follow her cues and channelled routines she lays out for your physical cleanse that precedes the astral opening of the pineal gland. However, once again you are cautioned against the blind falling into the pit of the 3rd Eye, and you should be prepared to get exactly what you asked for, or even more.
HP is the spirit of Ayahuasca and DMT, respectively, and she should be treated with the utmost respect and professional guidance if you want to avoid more precarious and unpleasant visions and spiritual experiences.