Obsidian Daggers were handed out to members of the Shadowlords Faction as part of the Cloak and Dagger event, which was part of the Salvation Chapter of the Warriors of Destiny event cycle.
They are very similar to Glass Daggers. They appear to largely have served the purpose of helping the Shadowlords' followers slay Clainin's Paladin guards.
Each member of the Shadowlords can get two Obsidian Daggers per day from the display case in the Shadowlords Sanctuary. The have a countdown timer on them and decay after 7200 seconds.
For more detail, see the article on the Cloak and Dagger event.