Wolfgang is absolutely right. The double-edged dagger is a level 2 object used in the path to Ta-Seti
This object is played before choosing Battle cards. If you win the battle, you gain one additional Victory Point. That means two VPs for one battle. You take one regular VP token (a red one) and one double-edged dagger token (using the front side).
If you lose the battle, you don't take any regular VP (of course) and you take one double-edged dagger token (but using the flip side with the -1 VP symbol)
So if you use it, you have to be sure to win the battle ! :-)
This begs the question of do you get the extra point even if you lose all your troops but win the battle? Normally you wouldn't get the battle VP but it seems like you should get the one for the dagger. Not sure how it goes officially?