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It's a channeled execute, and the last skill in the Dawn's Wrath skill tree. Morph 1 is Radiant Glory and heals you based on the damage you do. Morph 2 is Radiant Oppression and does up to 40% extra damage scaling off of how much magicka you have at the time.
I took the Radiant Glory morph, thinking the healing might be a good thing to have, but to be honest, it's so insubstantial that it's not worth it. I'll probably respec and do Radiant Oppression. Plus, the channel animation is different and looks cooler than that of Glory.
Yeah, people thought it's op because many players stay low on HP while in battle (spamming absorb shields as healing ward, or to trigger set-item effects, etc. etc.) so executes do full dmg. Since this skill is a "sequence of low dmg executes" that do 5-7k dmg per hit, it can result in up to 30k dmg, if the target is at low-HP during the 3 second channel. To explain, why exactly many classes stay around 30% hp in battle would require too much time