Barrel Replica

The mystic code possessed by Sion. A replica of the Black Barrel, one of the seven superweapons on display at the Atlas Institute. It is a conceptual armament of “natural lifespan,” which demonstrates a toxin (attack power) in proportion to the predestined span of the target's life.
Sion uses this Barrel Replica in her Arc Drive, but it seems that she isn't utilizing its original function.
Also, though she discharges the energy horizontally in her Last Arc, Barrel Replica Obelisk, she is able to avoid being blown back by the recoil by anchoring her body to the ground with Etherlite.

MB Act Cadenza PS2 Manual: Tsukihime Dictionary