The Shop's Recipes and Knowledge section offers ten Master of Death Tomes. After purchasing and studying one of them, you will receive the ability to instantly kill Common Monsters of a certain level range.

When you left-click on a monster whose level corresponds with the Master of Death Tome you studied, an additional Kill option will appear under the Attack button.

When instantly killing monsters, the Loot you receive will be of the same amount and type that you would have gotten if you had chosen the normal Attack action. 

Please note that the Kill command can't be used on:
• Elite monsters;
• Event monsters;
• Instance monsters.