Atonement is a Discipline Priest talent that let's them heal through dealing damage (Smite and Holy Fire). While in a raid environment, the healing throughput is generally not even close to competitive to a pure healing spec, this is one of the rare occasions where atonement really shines.

I want to thank my guild for providing me with this opportunity. As you can imagine, as a healer I'm not really at the top of the priority list for a damage caster legendary.

Here's a link to my spec: http://eu.battle.net/wow/en/character/wrathbringer/Karal/talent/secondary