BioWare community spokesperson Chris Pearson offered up the following details:
Hawke meets a mysterious female Elvin assassin named Tallis, voiced by the talented Felicia Day, and joins her to steal an ancient relic from an Orlesian Baron in his well-guarded estate. A cross between Varric and Isabela, Tallis is a rogue who introduces a new fighting style to add more variety in combat. Get up close and personal with her deadly skills or stand back and pick off enemies with her ranged combat abilities.
Mark of the Assassin features new items, new enemies and monsters and new environments as well as giving players the opportunity to experience a new, tactical way to play with the return of Stealth gameplay. Plus, along with the new environments and monsters, you will learn more about the Orlesians and Qunari.