Halo 2 was the first mainline Halo game to make the Energy Sword a usable player weapon.
The weapon had appeared in Halo: Combat Evolved, but players could not pick it up and use it as a regular weapon there. In Halo 2, the Covenant Type-1 Energy Weapon became available for player use, giving the series a powerful close-range option associated especially with Elite enemies and the Arbiter. Its inclusion became one of the sequel's memorable gameplay additions.
The Energy Sword is more than a standard melee weapon: it has a limited charge, can perform a lunge attack toward targets, and occupies a weapon slot. Those mechanics helped establish the distinctive close-quarters style later seen throughout the series.
Some players remember the weapon as originating in the first Halo because it is visibly present during Combat Evolved. The important distinction is between appearing as an enemy-held object and being usable by the player. Halo 2 introduced the latter in the mainline series.