The caster can use this spell to summon a single Greater Daemon. The creature appears within 6 yards of the caster. However, this spell may only be cast if the Daemon has already been summoned, controlled, and bargained with - according to the rules given under Summoning Daemons.
Once the Daemon has been summoned and successfully controlled, the Daemonologist may set it one task. The Daemon will remain until the task is completed, it is dispelled, or until it or the summoner is slain.
Note that "Go away and never bother me again" is one task.