Code:
new zombie;
zombie = CreateEntityByName("tf_zombie_spawner");
if (IsValidEntity(zombie))
{
DispatchKeyValue(zombie, "targetname", "SkeletonKing");
DispatchKeyValue(zombie, "origin", "X Y Z");
DispatchKeyValue(zombie, "angles", "0 0 0");
DispatchKeyValue(zombie, "zombie_lifetime", "0");
DispatchKeyValue(zombie, "max_zombies", "1");
DispatchKeyValue(zombie, "infinite_zombies", "1");
DispatchKeyValue(zombie, "zombie_type", "1");
DispatchSpawn(zombie);
AcceptEntityInput(zombie, "Enable");
}
Set infinite_zombies to 1 will make skeleton king respawn in origin after it die.