Warner Bros. Interactive have signed an agreement with Epic Games to use Unreal Engine through 2014. The "long-term studio-wide license" will allow any game published by Warner Bros. to make use of Epic's technology and two Warner studios, Rocksteady Studios (Batman: Arkham City) and NetherRealm Studios (Mortal Kombat), will exclusively make use of Unreal Engine for all upcoming titles.
It remains to be seen if other studios under the Warner umbrella, such as Monolith Productions, will make use of Unreal Engine in the future or if they will continue to use their own propriety technology.