Curved knife held in the right hand of the dakini, from a tangka of Yeshe Tsogyal
The 'curved knife' (Tib.གྲི་གུག།, Wyl. gri gug) is a symbolic instrument that symbolizes the severing of the four demons. 
↑*bskyed rim gyi zin bris cho ga spyi ’gros ltar bkod pa man ngag kun btus/ kun mkhyen bstan pa’i nyi ma. Quoted in the glossary of Deity, Mantra & Wisdom, Translated by Dharmachakra Translation Committee.