First time playing - I've read through the instructions (3 times) and it mentions MONSTERS - how do the monsters come into play? The instructions talk about monsters having turns and their abilities, but no where does it say how monsters come into play.... please help!
Once you fail a Haunt Roll (roll 6 dice, if you roll less than the number of Omens drawn so far, it is a fail), the haunt starts.
Monsters will come into play by specific instructions once the Haunt starts, each Haunt is completely different from one another, it may be that the Traitor can spawn Monsters or that you start the Haunt with Monsters already in play, alternatively, a Haunt may not involve Monsters at all. In any case, the Haunt instructions will tell you.
You can check "The Haunt" section in UltraBoardGames' guide for more information.
Monsters are a haunt related thing, and it will tell you exactly how and when they enter usually in the Traitor's Tome. A haunt is triggered when someone rolls less than the number of Omens that have been drawn this game on 6 dice, so anywhere from omen 1 to omen 13 can trigger the haunt.
Take haunt 1 for example, the Traitor's Tome says:
Put the large circular Mummy token and a pentagonal item token (representing the sarcophagus) in the room with you.
So the Traitor will follow these instructions to put the monster(s) into the game when the haunt begins. In haunts with monsters but no traitor, the instructions will be in the Secrets of Survival book. Like Haunt 12 where it says:
Put a number of small monster tokens (representing Evil Twins) in the Entrance Hall equal to the number of players. Use tokens of different colors, each corresponding to a specific hero.
Some haunts have no monsters at all, just the Heroes versus the Traitor, like Haunt 9. Which haunt you get depends on what omen was drawn when the haunt roll failed and what room it was drawn in, there's a table at the beginning of the Traitor's Tome. In BaHotH anything that is not haunt specific is put in the rule book, that includes the basics of monster turns and interaction with monsters (usually combat for the heroes) but things that are haunt specific can't be in the rule book, because they are supposed to be secret and have only a small chance of applying to that game.