Custom NPC spawn

[Keerok]

Hi, i am pretty new to making custom scripts, just wondering how to do the following if possible

I would like a npc to shout in zone if another rare pop npc spawns rather the the actual rare shouting if it spawned
NPC A = watch npc
NPC B = rare spawn

I have the watcher set to invis/untargetable etc and the rare npc set to a low chance to spawn, just trying to figure a way to make the watcher shout if it spawns

Or if there is a better way to do it -)

Thanks for any info