Exhaust this card when a monster enters an empty space within 3 spaces of a trap token to immediately perform an attack targeting that monster. After the attack is resolved, if the monster was not defeated, it may continue its activation.
Q: When using a Stalker's Ambush ability at the same time as a monster moves adjacent to a trap, would both abilities trigger or would the OL be able to choose which to activate first (I.E. the trap), and making Ambush invalid?
A: If this happened, Ambush would still trigger. The timing conflict will merely reflect which will happen first. The trigger, a monster entering a space adjacent to (or within 3 spaces of) a trap token has still been fulfilled for both.
Source Justin Kemppainen, Creative Content Developer, Fantasy Flight Games