Naegling was Oromis' Rider's sword. It was a bright gold-bronze, closely matching Glaedr's golden scales. During their years in Du Weldenvarden, Oromis and Glaedr stored every bit of their excess energy into the gem, a yellow diamond, on the pommel, as well as several elves that came twice a week to pour as much of their lifeforce as they could into the gem. For example, Oromis stated in Brisingr that there was enough energy within Naegling to "shift an entire mountain". Oromis used Naegling when dueling Eragon and had it at hand when Glaedr returned from his fight with Saphira. This Sword was lost during the battle between Oromis and Murtagh resulting in Oromis' death, as Oromis had placed wards around it to protect himself.
The sword may have been taken by Murtagh after the battle and taken back to Urû'baen to be put in Galbatorix's collection of swords from the Dragon Riders he'd murdered. If so, it may have been recovered from Galbatorix following his death. However, it was probably retrieved by the elves and kept for safekeeping.