Read our Wiki article about areamonster. Syntax:
areamonster <"map name">, <x1>, <y1>, <x2>, <y2>, <"display name">, <mob id>, <amount>[, <"event label">];
Example:
areamonster "prt_fild08", 50, 50, 100, 100, "Poring", 1002, 10, strnpcinfo(3) +"::OnPoringDeath";
Spawn 10 Poring (1002) named "Poring" in a rectangular area between coordinates 50, 50 and 100, 100 on the map "prt_fild08". When it is killed, run the event label "OnPoringDeath" in (this NPC name).