Oathbringer was the name given to the Shardblade held by Brightlord Dalinar and later by Brightlord Sadeas.
History
Oathbringer was Bonded to Dalinar when he was "twenty Weepings old" and was made it's first appearance in the Way of Kings when Dalinar was fighting a Chasmfiend and protecting Elhokar. He uses it in combat with the Parshendi and eventually gives the blade to Sadeas.
Physical Appearance
The blade was six feet long, slightly curved and a handspan wide. It had wavelike serrations near the hilt, and curved like a fisherman's hook near the tip.
Trivia
Throughout the novel, Oathbringer is referred to by name, however when Dalinar summons it to give to Sadeas, it is simply "The Blade" perhaps implying that his perspective has changed and he no longer cares for fighting.