Complex sentence: Forge, a level editor that first appeared with "Halo 3", was updated for "Reach", and can be used to edit the default multiplayer maps as well as a large blank map known as Forge World.
Split into simpler sentences: Forge, a level editor that first appeared with "Halo 3", was updated for "Reach". Players can edit the default multiplayer maps as well as a large blank map known as Forge World.