You gain a +1 bonus to attack and damage rolls made with this magic weapon.
In addition, while attuned to the dagger your sneak attack dice increase from d6 to d8.
Transfixion was payment to the assassin, Jedik Silvercrow, for payment from Bhaal for the murder of Thay's Zulkir of Abjuration nearly 1100 years ago. It has remained within the Silvercrow family, until Bree lost in the Great Gray Land of Thar. Rumors persist that the dagger was used to complete the God of Murders resurrection.