The A'tar Blade was a melee weapon used by Goa'uld Ashrak assassins. They were a forged Naquadah blade that was capable of penetrating any form of armor.
History
The knife used by an Ashrak who managed to get to the Tau'ri' s Alpha Site. He used the knife to kill Tok'ra, Jaffa, and Tau'ri. After the Ashrak was dead, Bra'tac hailed the knife as a symbol of the alliance between the Tok'ra, Jaffa and Tau'ri, because like the Goa'uld it had brought them together against a common enemy. (SG1: "Allegiance")