Nero's right arm in Devil May Cry 4. Its origins remain a mystery but it bestows upon Nero incredible power, including the ability to bend time and space to pull himself across distances instantly and pull enemies to Nero. It also houses the Yamato when he's not drawing upon its power.
It is a part of Nero's demonic heritage, and taps into his desire for power. He would go on to lose his arm to his father Vergil ripping it off in Devil May Cry 5.