There's another way to spawn an emperium since it requires a guild castle database entry in order to be spawned. Simply make a duplicate mob that mimics it.
This is the original Emperium mob database entry:
1288,EMPELIUM,Emperium,Emperium,90,68430,0,0,0,1,60,71,40,50,1,17,80,50,26,20,10
,12,0,8,26,0x120,300,1288,288,384,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
0,0,0,0,0
Now create a new mob that is identical to it (chosen mob ID number is an example):
3024,EMPELIUM_2,Emperium,Emperium,90,68430,0,0,0,1,60,71,40,50,1,17,80,50,26,20,
10,12,0,8,26,0x120,300,1288,288,384,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
0,0,0,0,0,0
Lastly in mob_avail add the following so your new mob matches the same sprite as Emperium:
3024,1288
Now you can create a permanent monster respawn using the command Arcenciel has stated for the custom monster mimicking an Emperium without any further database edits.
Source of Info: http://www.eathena.ws/board/index.php?showtopic=254539