The event
The Union of Maedhros is broken and the Elven realms of Beleriand are ruined.
Causes
An attempt to overthrow Morgoth by an alliance of Elves, Men and Dwarves.
Consequences
Húrin is captured and cursed; Gondolin and Doriath are left isolated.
Sources
The Silmarillion · 1977
Posthumously edited. Contains the Ainulindalë, Valaquenta, Quenta Silmarillion, Akallabêth and Of the Rings of Power.
The Children of Húrin · 2007
A continuous narrative assembled from earlier drafts.