The Phoenix: Rettenjo is the ninth Nox Nyctores that was created during the Dark War in the shape of 55-inch nail. It is designed as an anti-Nox weapon and was the last of the original Nox's that were created during the war itself.
After discovering Kushinada's Lynchpin, Nine the Phantom had it taken to her laboratory underneath the Magister's City of Ishana. There, she removed its core and created the Phoenix: Rettenjo, a large nail which was to be used to destroy the other Nox Nyctores after the end of the Dark War. It was not designed to combat the Black Beast. After the war ended, however, it went missing.
Nearly 80 years after the war had ended, Rettenjo had found itself in the possession of Tenjo Amanohokosaka. She refused to use it in conjunction with the Kushinada's Lynchpin during the Ikaruga Civil War and sealed her soul within it before giving it to Bang Shishigami to protect it.  Bang was never told the details of the weapon, but eventually nailed it into the Lynchpin, sealing off the seithr from the world. It remained within the Lynchpin in the world created by the Embryo while another version of it was in Bang's possession. Ragna the Bloodedge took the wish of Tenjo while her soul slept within the Rettenjo. It is presumed in the new world created by Ragna that the Rettenjo still rests within the Lynchpin, as it was not seen on Bang's back in the finale of BlazBlue: Central Fiction. 
The Nox takes the form of a 55-inch nail with a wide circumference. Underneath its head is tied a bright red scarf. Rettenjo has the properties of being able to stop the flow of seithr and destroy the other Nox's that exist.  Other than its physical abilities as a melee weapon, it has little to offer in the way of combat.
Nox Nyctores number 9. Possessed by Bang.
It was once possessed by Tenjo. It's also known as the 55-inch nail. It was developed by one of the Six Heroes, Nine, as the core of 'Kushinada's Lynchpin', and an 'Anti-Nox Nyctores' weapon. It has the power to destroy other Nox Nyctores. 
A Phoenix is a mythical Greek bird that rebirths itself after each death by resurrecting out of its ashes.
- BlazBlue: Spiral Shift - Hero of the Frozen Blade, Chapter 4
- BlazBlue: Central Fiction, Story Mode, Episode 012, Chapter 100
- BlazBlue: Chrono Phantasma, Arcade Mode, Bang Shishigami - KUSHINADA
- BlazBlue: Central Fiction, Library Mode, Weapon #009