It can be obtained from opening the Treasure Hunter interface during the promotion, or from Diango as an ironman. Obtaining one prompts the message You have been given a[sic] Assassin's Contract list! Use it to complete contracts and earn some sneaky rewards!
For the rewards and tasks, see Faceless Assassins.
After the event, if the player attempts to open the list of tasks from their inventory, the item will be automatically destroyed and a game message will appear in the player's chatbox: This item has no more use!