It's a little hard to summarize what would be helpful for you without knowing your experience level with the editor. I am working on a tutorial to help demonstrate how you start from scratch to
implement a basic quest. The fifth installment deals with using a dialog flag to trigger an item being given to a player, and that is essentially the same kind of paradigm you could use, just using different story calls.
In short you could:
- Create something in your level to serve as the spawn point, or place the monster in your level and set it off stage.
- Trigger an event using a dialogue flag or OnUse event from an item
- Catch that event in a story script and call a procedure to "spawn" the monster.
There is usually more than one way to do things and this is a bit of an open ended question but perhaps this gives you some idea as to how to approach the issue.