Well it's pretty straightforward:
//script.c
static int buildin_areakill_sub(struct block_list *bl,va_list ap)
{
status_kill(bl);
return 0;
}
BUILDIN_FUNC(areakill)
{
const char *mapname;
int16 m;
int x0, y0, x1, y1;
mapname = script_getstr(st,2);
if ((m = map_mapname2mapid(mapname)) < 0)
return SCRIPT_CMD_FAILURE;
x0 = script_getnum(st,3);
y0 = script_getnum(st,4);
x1 = script_getnum(st,5);
y1 = script_getnum(st,6);
map_foreachinarea(buildin_areakill_sub,m,x0,y0,x1,y1,BL_MOB);
// If you want to kill players as well, add BL_PC:
//map_foreachinarea(buildin_areakill_sub,m,x0,y0,x1,y1,BL_MOB|BL_PC);
return SCRIPT_CMD_SUCCESS;
}
// def
BUILDIN_DEF(areakill,"siiii"),
I'm not sure if you wanted to kill mobs or players, or both.