Chirugai

The  is Kurtis Trent's mysterious weapon. It is an heirloom from the Lux Veritatis.

When Lara returns to the Arena where Kurtis fought mutated Kristina Boaz. She finds it on the ground, but no sign of Kurtis. When she picks it up and holds it in the air, it starts vibrating, which probably means it is pointing to Kurtis, who probably still alive since his hit from Boaz.