To add a custom monster summon branch, the steps are:
Add MOBG_G_D_Branch_01 before MOBG_MAX in https://github.com/rathena/rathena/blob/2f311bd1ef6abf0de642f93690eb37bebbb34d14/src/map/mob.hpp#L116
Add MOBG_G_D_Branch_01 after export_constant(MOBG_TAEKWON_MISSION); in https://github.com/rathena/rathena/blob/2f311bd1ef6abf0de642f93690eb37bebbb34d14/src/map/script_constants.hpp#L4625
After compiling your server should have created a folder named "import". Add your custom branch data in import/mob_summon.yml
Header:
Type: MOB_SUMMONABLE_DB
Version: 1
Body:
- Group: G_D_Branch_01
Default: MAYA
Summon:
- Mob: MAYA
Rate: 100000
- Mob: DEVILING
Rate: 700000
- Mob: Drake
Rate: 700000
4. In import/item_db.yml
- Id: ****
AegisName: name
Name: name
Type: Usable
Buy: 10000
Weight: 200
Flags:
BuyingStore: true
DeadBranch: true
Script: |
monster "this",-1,-1,"--ja--",-1-MOBG_G_D_Branch_01,1,"";
All other steps are unnecessary (I think you just forgot to compile according to the error in your first post).