Not a Decepticon
Hero
I by general rule ignore his twitter posts because it has been very much a meme he's been often blatantly wrong, like ruling Paladin cannot divine smite with unarmed strike because unarmed strike is not a weapon attack, even when it is classified as a melee weapon attack.Well that was my thought, when a random encounter with incorporeal enemies came up. But Jeremy Crawford says otherwise, apparently, and I was at the time forced to accept his "ruling".
The enemies can just set up the ambush behind the next door the party has to go through, they have 8 hours to see the hut and organize it, they don't need to wait directly in front of exit from the hut.As for the ambush, well, consider that while the sphere grants you and your party total cover, you can see outside of it just fine. The spell only ends when the caster steps out of it, and nothing stops them from casting a new one if they see a mass of enemies forming.
So the ambush would require the whole party to not notice the enemies massing until the ritual ends, because otherwise, they have a perfect defensive position to work with. Further, any wise Wizard will also stock alarm. My Wizard PC does, and used both just in case (I might have issues with LTH, but as a player, I'd be an idiot not to use it).