The Knights Phantom (aka the knight phantoms), during the Last War, were specialized light cavalry that used phantom steeds to reach the critical point in a battle, then a combination of spellcasting and martial prowess to win the day. Part of Aundair's famed Knights Arcane, the knight phantoms prided themselves on their ability to ride hard and reach strategic points in the battle before the enemy could--or to rescue Aundairian units in danger of being overwhelmed by the enemy.
Like other members of the Knights Arcane, the knight phantoms remain in service to the Aundair crown after the end of the Last War. But their numbers are diminished. At the height of the Last War, the knight phantoms were able to field two heavy cavalry companies for the Knights Arcane--nearly 400 knight phantoms in all. Now only a single company of 150 knight phantoms remains on active duty; the rest are dead, retired, or on long-term leave from the Knights Arcane. Many of those on leave or "detached duty" have put their war-won talents to use as adventurers.
Knight phantom is a prestige class that is available only to 5th level wizards, 6th level sorcerers, and 7th level bards. The latter two classes are relatively rare, because few characters expend a precious spells known slot on phantom steed-the signature spell of the knight phantoms.
Note that Five Nations conflicts with itself on whether knight phantoms are light cavalry or heavy cavalry. They are referred to as heavy cavalry only once however, and light cavalry several times.