Betrayal at House on The Hill, 2nd Edition - Monsters
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.