Made me think of NPC's spawning during dungeoneering. So I looked up a Dungeoneering base.
Make something trigger this to happen:
I'm sure you can tweak it to your needs.
Code:
public void loadDungeonSpawns() {
Server.npcHandler.spawnNpc(c, 111, 3313, 5482, 0, 1, 100, 10, 800, 200, false, false);//ice giant
Server.npcHandler.spawnNpc(c, 111, 3303, 5485, 0, 1, 100, 10, 800, 200, false, false);//ice giant
Server.npcHandler.spawnNpc(c, 116, 3301, 5477, 0, 1, 100, 10, 800, 200, false, false);//cyclops
}