The Protoss Arbiter is one of the more interesting of the "spell casting" units in Starcraft, making it similar to the Zerg Queens and Terran Science Vessels. It can be constructed at a Protoss Stargate after an Arbiter Tribunal has been built. It is the only Protoss combat unit not controlled by the Templar caste, instead it is flown by Judicators with their tremendous psionic abilities. Thus, in one of the single player Protoss missions in the Brood War expansion (the mission to crush the rebellion led by the Judicator Aldaris) you are unable to directly build these potent units. It costs 100 minerals but also 350 Vespene Gas units, and so qualifies as a late game unit.
Without any upgrades the Arbiter already is capable of doing fairly interesting things. All friendly units in close proximity to the Arbiter become cloaked (except for other Arbiters), rendering them invisible to the enemy unless there is a detector unit nearby. Upgrades at the Arbiter Tribunal provide for two interesting spells: recall and stasis field. The recall spell can be cast at a group of friendly units, anywhere on the map, and this will immediately teleport them to the Arbiter's present position, making it useful for supporting hit and run attacks, bringing ground units to difficult-to-reach spots, and rapidly bringing reinforcements to a besieged position. The stasis field spell is similar to the Lockdown ability of Terran Ghosts, except that any units caught in a stasis field cannot attack or be attacked, it hits multiple units in an area of effect, and it works on any unit (locked down units can only be mechanical, and can be attacked during lockdown). This is useful for neutralizing a surprise attack (stasis them then bring in reinforcements) and for reducing the number of enemies that can fight to weaken an otherwise overwhelming attack.