The Single-Button Assistant is a new spell you can click-and-drag to your action bar from the top right of your Spellbook and then use just like you would any other spell on your action bar.
This Single-Button Assistant will cycle through a prioritized list of abilities that your character knows and automatically use them against your current target. To see the full list of abilities that your current specialization will use, you can right-click on this ability while looking at the top right of your spellbook to expand or collapse the list of spells that will be cycled through. Using this Single-Button Assistant will impose a penalty to your Global Cooldown (GCD).
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