Use a monster, danger, or location move
Every monster in an adventure has moves associated with it, as domany locations. A monster or location move is just a description of what that location or monster does, maybe “hurl someone away” or “bridge the planes.” If a player move (like hack and slash) says that a monster gets to make an attack, make an aggressive move with that monster.
The overarching dangers of the adventure also have moves associated with them. Use these moves to bring that danger into play, which may mean more monsters.