expression, godan verb (-su)
dispatch a messenger; send an envoy
A formal or literary expression for sending a messenger or envoy on a mission.
王は隣国に使者を遣わした。
The king dispatched a messenger to the neighboring country.
使者を送る is a more common, everyday way to say 'send a messenger', while 使者を遣わす is more formal and literary.
A fixed expression composed of 使者 (messenger) + を + 遣わす (to send, dispatch). 遣わす is a classical verb that survives mainly in formal or literary contexts.