Misread your post, sorry. Let’s try that again.
You use a timer as the basic mechanism. The syntax for the timer is:
The time when Dangerous Arrival occurs is two turns from now. (This sets a timer called “Dangerous Arrival occurs”. You can call the timer whatever you like, of course.)
Personally, I’d use a scene to set it in motion:
Preparation Time is a scene. Preparation time begins when the player is in the Arena.*
When Preparation time begins:
(tab)The time when Dangerous Arrival occurs in two turns from now;
When Dangerous Arrival occurs:
(tab) now the Perfidious Agent is in the Arena;**
* obviously this can be any test you like: e.g. “when the player has taken the Brooch of Doom”
** or “now the Perfidious Agent is in the location of the player” if you don’t want the player to escape!
Hopefully this answer is more helpful now I’ve actually read your question! (Duh!)