The Marchwardens were Sindar (Iathrim) in Thingol's army assigned to guard the borders of the forest of Doriath, including such places as Aelin-uial. Their chief was Mablung of the Heavy Hand. For a while, Túrin joined them in the north-marches to wage war upon the Orcs.
After the Fell Winter, Marchwardens encountered Elves of Nargothrond who had survived the disastrous Battle of Tumhalad and sought refuge in Doriath. The Marchwardens led them to Thingol, and they brought 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 with them in disguise.