Halo: Campaign Evolved introduces three new bonus missions that precede the events of the original Halo campaign, featuring Master Chief and Avery Johnson. These missions are presented as a separate narrative, offering players the flexibility to tackle them before, after, or even during the main story.
In addition to the bonus missions, the remake includes new terminals and 32 extra skulls, which are collectible modifiers that can alter gameplay. These skulls can be found in all missions, including the bonus content.
Despite their chronological placement as a prequel, playing the bonus missions before the main campaign is generally not recommended. The bonus missions largely function as a standalone story with minimal direct impact on the original campaign's plot. A more significant consideration is their approach to weapon distribution. The bonus missions provide access to nearly every weapon within a short timeframe, a stark contrast to the main campaign, which gradually introduces weapons. This allows players to experience each weapon individually as it's unlocked in the core campaign. The rapid influx of all available weaponry in the bonus missions can diminish the impact of weapon discovery as a core part of the original game's experience.
However, there is one specific reason to consider playing the bonus missions first: the Grunt Birthday Party skull. If a player is keen on experiencing the game with this particular modifier active from the outset, completing the bonus missions early would be the way to acquire it. Otherwise, for a more cohesive experience with weapon progression and narrative pacing, proceeding through the main campaign first is the advised approach.