Included in Rift’s Starfall Prophecy update is the option to upgrade specific abilities to legendary status. You get your first legendary ability at 66 and the last one at level 70 for a total of 5. Furthermore, certain souls can have a certain amount of legendary points spent in them. For instance, in the video, the Warrior can spend 3 points in Reavers, and 1 each in Riftblade and Warlord.
Legendary Abilities will amplify the original ability great, either by increasing its damage (or healing) or adding a stun, or various other benefits.
Check out the video or hop into Rift for a full run-down.