Open up src/map/mob.c and find the following code:
const char* mobfile[] = {
DBPATH"mob_branch.txt",
DBPATH"mob_poring.txt",
DBPATH"mob_boss.txt",
"mob_pouch.txt",
"mob_classchange.txt"};
Edit it to this:
const char* mobfile[] = {
DBPATH"mob_branch.txt",
DBPATH"mob_poring.txt",
DBPATH"mob_boss.txt",
DBPATH"mob_superboss.txt",
"mob_pouch.txt",
"mob_classchange.txt"};
Now you go into your db/pre-re OR db/re depending on your settings and create a new text file with the name
mob_superboss.txt
The db uses the following structure
MobID,DummyName,Rate
Example:
1708,Thanatos Phantom,1000000
1734,Kiel D-01,1000000
2022,Nidhoggur's Shadow,1000000
Now you open src/map/mob.h and change the following to 6
#define MAX_RANDOMMONSTER 5
Looks like you changed the item code already so you're ready to go.