Balor's Death Throes: Burst or Spread?

[DreamGoddessLindsey]

Topic.

I've been running it like a spread, which has saved some people from dying outright, but then I realized that it isn't said which it is.

[Bizbag]

It's described as a "flash" of fire, so I'm going with a burst. This'd let a clever party take cover before they struck the final blow and avoid the damage; perhaps everyone takes cover. Assuming the demon is Anchored and at low HP, the wizard could delay, the party take cover, and the wizard cast Time Stop and set off the final attacks to destroy him.