... So I've sat down to actually do it, and I've noticed a few things worth mentioning: Obviously, the 40' wall can't wrap all the way around the 20' radius sphere. This would be why tglassy mentioned wrapping it back on itself, I assume. So it's only going to be effective in stopping them from running FORWARD, not from running away. Of course, they barely know what's going on.
Another note on Hunger of Hadar: This is the kind of spell that would give PTSD to people who see it in use. It might be powerful, but it's very likely to colour the way NPCs look at the caster (and the party in general). I can see people having an easier time justifying the Necromancy, which is notoriously unpopular itself.