Aragorn has asked you to find the bodies of many Rohirrim who fell in combat, and to search for Nothmar, who was last seen making towards the city.
'Éomer has provided me a list of names of those known Rohirrim who were slain by the enemy on the Pelennor.'
You explain that you saw Nothmar on the Pelennor with Herefara and Herubrand.
'It seems that Nothmar is still missing. We thought he was in the city but word has come back that he never made it. Please look for him while you are out there. He may yet still be alive.'
Grimbold, Déorwine, Dúnhere, Guthláf, Herefara, and Herubrand died on the Pelennor. Nothmar is missing.
Search for the valiant Rohirrim on the Pelennor Fields, after the battle.
You should talk to Aragorn, who can be found at his pavilion on the after-battle Pelennor.