The Marchwardens were Sindar (Iathrim) in Thingol's army assigned to guarding the borders of the forest of Doriath, including such places as Aelin-uial. Their chief was Mablung of the Heavy Hand. For awhile Túrin joined them in the north-marches to wage war upon the Orcs.
After the Fell Winter, Marchwardens encountered Elves of Nargothrond who survived the disastrous Battle of Tumhalad and sought refuge to Doriath; and led them to Thingol, bringing news from Nargothrond. Morwen then fled Doriath seeking news of her son, and Mablung with hardy Marchwardens were tasked to bring her back; unknown to them Nienor came disguised with them.