In Kingdoms of Amalur: Reckoning for the PC, PS3, and Xbox 360, once you fill the fate meter you can enter Reckoning Mode to unleash powerful attacks, and get a nice XP bonus as well.
First, to fill the fate bar, you need to mix up your attacks. Using melee, magic, and ranged attacks in a single battle will fill the fate meter notably faster than just using one type of attack.
Once the fate meter is filled, pull LT and RT at the same time on Xbox 360, or L1 and R1 on PS3 (or possibly L2 and R2, PS3 users please confirm as I have the Xbox 360 version).
Try not to save your fate meter for extra hard battles, as you will miss out on a lot of XP that you would receive from using it regularly.
