Isnt that borax?
That was my second thought, especially since Kevin super-soakers the boys with Borax earlier in the trailer. But the splash happens right when Kevin's voiceover says "banish all the demons off earth", which is curious timing.
And Kevin's girlfriend is reporting to Crowley, who is the king of all demons.
UNLESS
Unless she's a Leviathan AND CROWLEY IS NOW ALSO THE HEAD OF ALL LEVIATHANS TOO
ASDFGHJKL