When one of the Dhûr-Lhûg flies into battle on their mount, a heavy tolling of bells echoes throug the battlefield. They instill fear into anyone who hears it, as the bells have become synonymous with the havoc and destruction that is the wrath of the Dhûr-Lhûg.
The bells also serve as a symbolic honouring to the Dragon King, because it is them For Whom the Bell Tolls.