Aragorn has asked you to find Dervorin and Rossaran, both presumed to be missing in combat.
'Dervorin, son of Borhador, and Rossaran, a Swan-knight under Imrahil's command, are unaccounted for. You should search for them at once.'
Aragorn can be found at his pavilion on the after-battle Pelennor.
You should return to Aragorn now that Dervorin and Rossaran are found.