src\map\battle.h
inside
extern struct Battle_Config
{
.....
int item_restricted_consumption_type;
int max_walk_path;
int item_enabled_npc;
int multiplier; // add this
} battle_config;
src\map\battle.c
inside
static const struct _battle_data {
...
{ "item_restricted_consumption_type", &battle_config.item_restricted_consumption_type,1, 0, 1, },
{ "max_walk_path", &battle_config.max_walk_path, 17, 1, MAX_WALKPATH, },
{ "item_enabled_npc", &battle_config.item_enabled_npc, 1, 0, 1, },
{ "multiplier", &battle_config.multiplier, 0, 0, INT_MAX, },
};
and you can call it using battle_config.multiplier