Dozens of the creatures are burned to ash in the spell, but even as you do that, the remainder of them squeal and split up.
SpoilerEven with a save, the ones that died would still have.
Overall init on remaining Quasits: (1d20+7)[18]
Just a roll: (5d5)[11]
MW:Spoiler
Yes, you're still trudging on.
The quasits look around, and spot those of you who had not hidden. They do miss seeing Memnoth, who is still mostly buried in the sand, even after his yell.