Part 2 will see the final 5 stick figures do their challenging rooms, before the survivors then gather and take on a couple of very difficult joint rooms.
As for the boss... If you watched part 1, chances are that you already have seen it, as it shows its ugly face over 10 times there...đ
Because of how difficult this dungeon is, very few will make it to the boss, and even fewer will survive it. The reward to the survivor(s?) is an Att +1.