Hihiirokane is a sword that has been in the possession of Tomonori and Jūbei's family for generations and is said to come from the Age of Origin. When activated by its wielder speaking its name, it becomes a giant claw, and has the ability to cut both bodies and spirits, enabling it to injure Terumi. It was also used by Jūbei to cut a hole in the Black Beast for Celica A. Mercury to enter it during the final battle of the Great Dark War.
It was sealed in the Alucard Castle, in Clavis Alucard's coffin, and protected by Absolute Defense Type Zero: "Mirror's Edge". Later on it was retrieved by Terumi, who used it to completely sever his connection with Hazama, after which he absorbed it. After making it part of himself, he was able to manifest its power in the form of a chain with a golden head and green lines, and used it to shatter Hakumen's soul.
A copy of it was reproduced by Platinum the Trinity who materialized it by utilizing Tomonori's eye as an intermediary. It was given to Hakumen who entrusted it to Jin Kisaragi, who, as Susanoo, used it to cut down Terumi together with Ragna the Bloodedge.
In its unactivated form Hihiirokane appears as a large katana with a silver blade and a black scabbard. When activated it becomes a great green claw, larger than Tomonori himself.
- It is named after a legendary Japanese alloy. The alloy itself is used in the series as both the Murakumo Unit's armor and the golden metal on Hakumen's chest are constructed of it.