A Delvian Pa'u is a Delvian priest of the Goddess of the Delvian Seek. They possess a wide variety of psychic and empathic powers granted to them by their Goddess and focused through the mantras of the Delvian Seek.
There are multiple levels of Pa'us, numbered in ascending order from the first level on, but it is unknown what the highest level of Pa'u is, or even if there is one. It appears that a Pa'u is granted a new ability with every level of the Delvian Seek that they attain, but this is based only on visual, not vocal, evidence.
Pa'u Zotoh Zhaan, the primary Pa'u of the show, begins as a ninth level Pa'u, and ascends to the tenth level in the episode "Rhapsody in Blue." As a ninth level Pa'u, she has the ability to draw spirit portraits reflecting the inner nature of a person, render herself undetectable as long as she remains perfectly still, cause pain to another living being, force spiritual energy to become tangible, and of course, share "Unity" with another living being. When she reaches the tenth level, she gains the power to shield the mind of another being from psychological harm, proclaiming "I am now a Pa'u of the tenth level, able to protect!" as she defends Crichton from Pa'u Tahleen's psychic assault. However, while being held in a mental prison by Scorpius' mental clone, she finds herself unable to escape him. Scorpius comments, "Pity. A Twelfth [level Pa'u] could break this bond," hinting at the powers of said level.
Other Delvian Pa'us have shown the abilities to plant illusions in the mind of others and even shatter a person's sanity entirely. These abilities come from darker impulses that a Pa'u is supposed to eradicate from her/himself. The precise level and limitations of these powers is not known.