I assume that it's intended to be a nod to the original game's full title, Halo: Combat Evolved. But "combat" is an uncountable/mass noun:

https://en.wiktionary.org/wiki/combat

combat (countable and uncountable, plural combats)

And "campaign" is a countable noun:

https://en.wiktionary.org/wiki/campaign

campaign (plural campaigns)

So I guess it's grammatically incorrect without an article before "campaign".