The blade was first seen in the season two finale when it was used to kill Mianite. It seems to be the World Historian's main weapon
"invidere" is a Latin word. When translated to English, it means "begrudge". Which would make sense as World Historian stated that he begrudged the power that his children (the gods) wielded. Meaning that the weapon symbolized his envying of the god's powers. This is supported by the fact that the Kikoku is the only known weapon capable of killing a divine being.
The name also reflects Nvidia, the dark force which plagued the streamers throughout the both series.