Leader Powers are special attacks that each of the six leaders—the UNSC’s Cutter, Forge, or Anders, and the Covenant’s Arbiter, Brute Chieftain, or Prophet of Regret—possess. UNSC Powers are executed as attacks from the Spirit of Fire; UNSC leaders do not appear on the battlefield (except during Campaign mode). Covenant powers are contained with the leader and are used on the battlefield, directly from the leader. Proper use of a Leader Power can dramatically change the course of a battle.
For example, if Sergeant Forge’s army is being attacked by a rival UNSC force consisting of Flamethrowers interspersed with other troops, a well-placed Carpet Bomb can decimate the Marines before they can deal any damage to units vulnerable to Flamethrower attacks. A precisely timed Cryo Bomb from Anders can kill enemy aircraft and immobilize ground forces—and your own forces, so use with care! It is typically more dangerous and less effective to use the powers during a fight; there are less units for you to kill, and you won’t have softened up or slain the attackers, giving them more chance to whittle down your own troops.