The gameplay is heavily based on the multiplayer mode of Halo 3. However, through the installation of fan-made mod known as ElDewrito, it also contains a Forge mode where players can create custom maps. Halo Online is multiplayer-only and does not have a single-player campaign.
Halo Online takes place on Anvil, a secret UNSC space installation where both the Spartan-IV soldiers and the Sangheli (Elites) train for combat in simulated wargames with experimental weapons and armors.
