Batman: Arkham Origins will have online multiplayer. You're now free to debate the merits of this decision.
The multiplayer portion of Origins is called "Invisible Predator Online," and is being co-developed by Splash Damage, best known for their work on Brink. In this competitive mode, a group of three Joker thugs will be fighting a group of three Bane thugs. However, there's a wrinkle in what would otherwise be a standard team deathmatch affair: Batman, of course.
In addition to 3v3, another pair of players will control Batman and Robin, who must intimidate all thugs on the map. Variety matters for our masked heroes, as repetition will yield lower intimidation. The thugs, on the other hand, will simply try to kill the caped crusader and his sidekick.
Update: Siliconera points out that the mode will be available on PC, PS3, and Xbox 360--but not Wii U.
We understand your skepticism, but here, look at this trailer which proves we're not lying and it exists: