The Patrol Leader's Helmet is carried by the Svirfneblin Leader found with his patrol, during the quest Teach the Deep Gnomes a Lesson, a little south of the bridge to the Deep Gnome settlement in the Underdark.
One brings Phaere the helmet as proof of the deed. One can either slaugther them all, or ask Solaufein beforehand to stay behind, and reveal to the patrol you are sent to kill them to be handed the helmet over without bloodshed.
Description[]
| “ | This is the helmet of the svirfneblin patrol leader that you were sent to kill. It will act as proof that you have succeeded in your mission. | ” |