Bandersnatch sends out explosions that require players to know his patterns ahead of time, something that can be determined by talking to the residents of the shantytown for hints. Demoros, meanwhile, is not a boss you can kill; you have to beat him by rolling the dice and collecting coins that rain from above. The dungeon is available for players above item level 423, so get ready to take a chance and hope luck is on your side when you face the new dungeon next week.
Source: Official Site