Originally Posted by
bennyson
For those who are wonder why your Assassinate ability isn't instant killing their targets even though you have a high DC (50 - 60 DC or higher) It is most likely you have Venomed Blades active, this stance is bugged and is preventing Assassinate from working properly. Thus will not instant kill your target while scouting ahead for a group or soloing a quest. Even if the enemy is completely unaware of your presence.
To get Assassinate to work again, either turn off Venomed Blades or let party members or hirelings get aggro first while the stance is active.