Bartrand Tethras is a dwarven rogue and a hidden antagonist in Dragon Age II, though he begins as a somewhat gruff yet seemingly well-meaning figure whose betrayal does not take place until later in the game.
After gathering a party to explore the dangers of the Deep Roads with the aid of his own brother Bartrand aids the party and begrudgingly offers an even trade - waiting until they found an ancient artifact made from an exotic type of lyrium.
However upon getting his hands on the artifact and being exposed to the lyrium Bartrand's inner-greed was exposed and he lost his sanity - heartlessly locking his brother and the rest of the group inside the Deep Roads with the intent of letting them die, stating he had no desire to share wealth with any of them.
Although the group do escape (and gain considerable treasure as a result) Bartrand manages to evade justice for three years - during which tales spread of his growing insanity as a result of the cursed artifact: torturing his servants by forcing them to eat lyrium while he mutilated them and went on about dance and song.
Varric seeks to kill his now insane brother, if Anders is there he can temporarily restore Bartrand's sanity, if he does this then Bartrand expresses remorse for his actions and begs Varric not to leave him like this and not to allow House Tethras to meet such an undignified end. His ultimate fate is then decided by the player - if he is spared he is left at his estate temporarily before being sent to a sanitarum but if the choice is made to kill him Varric will kill him via shooting him with a bolt.
The artifact however is not to be found as Bartrand had already sold it to a woman (though in his madness he lost sense of logic , so he claimed she stole it).