Wait does the expansion come with away to play without the Overlord. I'm a bit confused here, first time I here anything about this.
Yes, it's a print on demand expansion (they release it as Gaming Night Kit earlier this year and now being available as Print on Demand). It comes with three new set of cards and new set of rules (that you have to download from their website). There is, what essentially the set-up deck, that picks the tile for the room, monsters, story and added certain conditions for the room (such as room full of fog and you can not exerted to move or monster do more damage etc.), the AI deck that dictate how monsters move and fight (since there is no Overlord player to control them) and small deck that basically there to prevent you from linger about too long (called Perils deck) and punish you if you do).
The rooms are unlike regular Descent is random so in the picture here you can see I just clear the first room and open the next room which turn out to be one of the "challenge room" which is similar to Act 1 in the regular game. My characters are pretty much ill prepared and probably will fail as I was hoping to get more gears before hitting this challenge but we will see. You can see the door to the right to the next room that won't get reveal until my character open the door so it stop players from able to plan ahead since you won't know what come up next.
You need just the base set and it doesn't use everything so it is somewhat limited but the set up deck do add variations to the lack of diverse enemies. There is a cool loot track mechanic that reward your kills but the way it works make you have to think about the order in which to kill the minions (pretty clever).
For the few times we play Descent, the problem is the Overlord seem to exist to just merely be the stumbling block for the players. it's not a real competitive play and lead to frustration for Overlord player and alpha gaming on the Heroes side.
They have another game night kit out and it seems to expand on this using the large expansion set along with the base game. I really wish they take this and make a full expansion with it as there are a lot of good mechanics.