STEPS TO MAKE A LOGIN BANNER

1] In first place you must create the script in /etc/rc.d folder :

# touch /etc/rc.d/banner
# chown root:wheel /etc/rc.d/banner
# chmod 755 /etc/rc.d/banner


2] Write the script, this will export TERM for clear the screen and print "Hello World". So edit the file /etc/rc.d/banner writting this lines :

#!/bin/sh
# PROVIDE: banner
# REQUIRE: cron bgfsck
. /etc/rc.subr          
name="banner"           
rcvar="${name}_enable"  
load_rc_config $name   
start_cmd="export TERM=xterm ; clear ; echo Hello World"
run_rc_command "$1"


3] You must enable the service, so edit the file /etc/rc.conf and add this line:

banner_enable="YES"


4] Finished. You just need to reboot and it would must run

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
The order of execution is showed with :
# rcorder /etc/rc.d/* /usr/local/etc/rc.d/*

You can change the order of your script with flags like REQUIRE, AFTER, BEFORE etc inside the script. And you have 4 big blocks called NETWORKING, SERVERS, DAEMON and LOGIN to use as checkpoints to set the order but for your purpose they are not useful. 
So if banner clears the screen too soon you can see the order of execution with the rcorder and add inside the script in the REQUIRE line the service you must execute before yours