This sketch might give us a clue.
Here we see a shockwave of devastating light radiating outward from a white-aligned human wizard. It's not clear why he's expelling this light so ruthlessly, but certainly it could wreak some carnage on a few nearby Nim.
Here's the answer, the relationship between the two images.
Retaliate art by Vance Kovacs
This is the final art of the card Retaliate, by Vance Kovacs. The concept behind the card is that the white wizard has emitted this holy shockwave to exact justice on some evil Nim that had been clawing at him. What we're seeing is that very moment of old-school, Leonin-style justice. Who says white doesn't have burn spells?