The Nicassar are a xenos species of powerful psykers who are allied with the T'au. The Nicassar are driven by an insatiable curiosity to explore and travel across the galaxy. It was this drive which led a flotilla of Nicassar Dhows to make contact with the T'au, resulting in the Nicassar becoming the first non-Tau race to become an addition to the fledgling T'au Empire.
Their contribution to the Greater Good is the provision of starships for the T'au Navy, the Kor'vattra, particularly vessels involved with reconnaissance and exploration duties, which are often crewed by the Nicassar themselves rather than the T'au Air Caste. The propulsion and navigation of all Nicassar vessels stems from the potent psychic powers that define the Nicassar species.
For this reason, the T'au have been careful to prevent the Imperium of Man, which is known for its extreme persecution of psykers, from discovering the existence of the Nicassar within their empire. The Nicassar are utilised by the T'au only for spacefaring; their own limited mobility makes them highly ill-suited for ground combat.
The Nicassar's extended families are semi-nomadic and are driven by a truly insatiable curiosity. This led to the original exploration of their home system, as they are only truly content when travelling. Because they can survive for long periods in virtual hibernation as a result of their alien physiologies, they have travelled far from their homeworld, albeit slowly.
First contact between the Nicassar and the T'au Empire came when a T'au Gal'leath-class vessel was setting up a Waystation in interstellar space and sighted a Nicassar flotilla that had been in space for centuries. As subjects of the T'au Empire the Nicassar must serve the Greater Good and do so by providing fleets of Dhows to scout and explore star systems on the fringes of the T'au Empire.
They are transported to their station by T'au vessels using gravitic hooks that are capable of interstellar travel through the Warp and commence a leisurely circuit reporting anything they find. When a Nicassar family travels, numerous Dhows travel docked together forming a larger community in which some members will be hibernating while others remain on watch.
Nicassar starships, known as Dhows, are relatively small star-faring yachts, propelled primarily by their captain's potent telekinetic powers, abilities which often determine who rises to positions of leadership in Nicassar society. Nicassar starships are primarily sleeper ships, in which the majority of the crew enter a state of hibernation that the Nicassar's alien physiology makes possible as the flotilla, consisting of multiple Dhows docked together, drifts between the stars, directed by the efforts of a few non-hibernating Nicassar crewmen.
Although poorly armed when they first encountered the T'au, the original weapons employed by Nicassar Dhows have been upgraded with Tau Railgun batteries. The T'au Empire uses Nicassar Dhows as Escort vessels for the T'au Navy, towing them across interstellar distances using the limited "Warp Dives" employed by T'au starships to travel across the galaxy.
- Battlefleet Gothic Armada - To Unite the Stars: Tau Vessels, pg. 107