Mablung, along with Daeron, came to the Mereth Aderthad bearing gifts and friendship from Doriath. He also served in Menegroth in some unknown position of command. Together with Beleg Cúthalion, he was one of the great captains of the Sindar, and were the only Elves of Doriath to fight with the Union of Maedhros in the Nirnaeth Arnoediad, as King Thingol would have nothing to do with the war.
Mablung was present when Túrin fought with Saeros over his taunting and restrained Túrin's wrath to avoid a brawl in the King's hall. Mablung then attempted to stop Saeros from taking his threats too far and warned him of Túrin's strength, should he attempt to fight him. Aware that a fight had broken out between the two, Mablung was present at the final play in the drama and tried to stop things from ending badly. It was to no avail Saeros ran despairingly in fear and fell off a brink where a stream ran into the Esgalduin, and struck his head on a rock and died. Later, Mablung was present before Thingol's throne where he accounted for everything that had gone on between Saeros and Túrin, and later accounts of Túrin’s choice to leave Doriath.
Mablung became captain of the guard of Doriath after Beleg left to seek Túrin. When Morwen Eledhwen, who dwelt in Doriath at the time, learned of the fall of Nargothrond and that her son Túrin might be there, she wished to seek him out. Mablung was assigned by Thingol to guard her on their way. Unknown to both Mablung and Morwen until too late, Nienor daughter of Morwen, had joined them. Mablung was unable to prevent them both from being ensnared in the traps of Glaurung, the dragon of Morgoth. Mablung hid from Glaurung and then searched the sacked Nargothrond, finding no sign of Túrin.
Nienor was found again by Mablung, in a trance, by the spell of Glaurung, at Amon Ethir. Mablung then led her back to Doriath, but he later lost her again when she ran off unclad started by a band of Orcs; a result of the spell of Glaurung. Thus she fled beyond the sight of Mablung. Unable to recover her Mablung returned to Doriath, but he spent years afterwards searching for Morwen and Nienor.
Mablung went to Brethil when he learned Glaurung was there, and there met Túrin. By admitting Nienor was lost he caused Túrin to realize that Níniel his wife had actually been his sister Nienor and Túrin killed himself.
Mablung was the last person to whom Melian spoke before departing Middle-Earth. She warned him about Thingol's Silmaril and to send word to Beren and Lúthien. Soon after Mablung was killed before the doors of the treasury during the Sack of Menegroth by the Dwarves of Nogrod.
- ↑ The Silmarillion, Quenta Silmarillion, Chapter XIII: "Of the Return of the Noldor"
- ↑ The Silmarillion, Quenta Silmarillion, Chapter XX: "Of the Fifth Battle: Nirnaeth Arnoediad"
- ↑ The Children of Húrin, Narn i Chîn Húrin, The Tale of the Children of Húrin, V: "Túrin in Doriath"
- ↑ The Children of Húrin, Narn i Chîn Húrin, The Tale of the Children of Húrin, XIV: "The Journey of Morwen and Niënor to Nargothrond"
- ↑ The Children of Húrin, Narn i Chîn Húrin, The Tale of the Children of Húrin, XVII: "The Death of Túrin"
- ↑ The Silmarillion, Quenta Silmarillion, Chapter XXII: "Of the Ruin of Doriath"
- ↑ The Complete Guide to Middle-earth