The Purifier (originally named Soul Hunter) was a ranged Protoss infantry unit that rode on a hovercraft. It had a ranged, arced lightning attack and the ability to gain power by killing Biological units. The Purifier was able to attack both ground and air units,and dealt additional damage to Biological units.
As the Purifier killed Biological units, its lightning attack got stronger and stronger, until it was able to shoot 3 separate beams, greatly increasing its damage output. This interesting mechanic was hard to balance, however, as the Purifier was extremely strong against Zerg opponents, who have many Biological units in their arsenal.
The Purifier was scrapped early in development.