What is the ESRB rating and genre of this game?

The game has an ESRB rating of T - Teen and falls under the Action and Strategy genres.

What new features does the DLC pack introduce for gameplay?

The DLC pack introduces the Changeling, a new Legendary Lord for Tzeentch, as well as new units, characters, and regiments of renown. It includes a new Legendary Hero, the Blue Scribes, and expands the roster with 5 additional units and 3 new Regiments of Renown.