A different variation of the action RPG formula was popularized by Diablo (1996), where the majority of commands—such as moving and attacking—are executed using mouse clicks rather than via menus, though learned spells can also be assigned to hotkeys. In many action RPGs, non-player characters serve only one purpose, be it to buy or sell items or upgrade the player's abilities, or issue them with combat-centric quests. Problems players face also often have an action-based solution, such as breaking a wooden door open with an axe rather than finding the key needed to unlock it, though some games place greater emphasis on character attributes such as a "lockpicking" skill and puzzle-solving.
