The Protoss Reaver has the most powerful* normal attack of all Starcraft units. It costs 200 minerals and 100 Vespene gas. A Reaver builds scarabs, which it releases in its attack, causing 100 splash damage (125 with an upgrade at the Protoss Robotics Support Facility), which is very effective for eliminating large numbers of enemies quickly. A single scarab will probably kill a large group of hydralisks instantly if aimed properly. Unlike the Terran Siege Tank (which in siege mode is the only other Starcraft unit with comparable attack damage), the Reaver can move even with this powerful attack, and has no minimum range (siege tanks in siege mode cannot hit units that come too close). The only drawbacks are that each scarab must be built by the Reaver at a cost of 15 minerals, and the Reaver can only hold a limited number of them (5 by default, 10 on an upgrade), the Reaver is very slow, making it vulnerable to fast moving units, and the scarabs can only hit ground units, making them highly vulnerable to air attack.
These strengths and weaknesses provide for one of the simplest yet reasonably effective strategies used by Protoss players: the Reaver drop. Take a Protoss Shuttle, load it up with two Reavers with the maximum number of scarabs, fly down to the enemy formations, drop the Reavers, watch them do their work, and then pick them up if/when the landing zone becomes too hot or if the Reaver runs out of scarabs. It's also best to accompany them with some air support such as a few scouts or carriers, to protect the Shuttle/Reaver combination from air attack.
* The infested Terran actually can do more damage, but that's a special ability, not an ordinary attack.
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