Halo 2 was the first Halo sequel to make the Energy Sword usable by players.
The Energy Sword appeared in Halo: Combat Evolved as a Covenant weapon wielded by certain Elites, but players could not pick it up or use it. Halo 2 changed that in 2004, allowing both the Master Chief and the Arbiter to carry the weapon during campaign missions and multiplayer matches.
Its gameplay impact was immediate. The sword is a close-range power weapon with a rapid melee attack and a forward lunge that can close distance to a targeted enemy. Its battery drains as it is used, so players must balance its devastating damage against limited energy. Halo 2 also gave players a new perspective by letting them control the Arbiter, an Elite who naturally fits the Covenant weapon’s history.
A common mix-up is saying the Energy Sword debuted in Halo 2. It actually debuted visually in Combat Evolved; Halo 2 was the first mainline game to make it player-usable. Later games, including Halo 3, Reach, Infinite, and Campaign Evolved, continued treating it as one of Halo’s signature power weapons.