The Phoenix Sword is a two-handed sword in NieR RepliCant and NieR Gestalt (NIER), found in the Shadowlord's Castle, after talking to the doves at the fountain, in a box at the left of the door you have to go through before encountering Devola and Popola.
The sword also appears in NieR Automata.
"A large sword used by temple guardsmen, and supposedly made from feathers of the legendary firebird."
This is an old story. A beautiful bird with brightly shining feathers lived silently and carefully in the depths of a forest.
One day, a child abandoned as a burden wandered into the depths of the forest. The bird took pity on the starved and sunken child, and pecked off one of its feathers to give to the child. The child brought it back and pleased his relatives, and he was able to live with his family again.
Hearing the story, people barged into the forest one after another, and told the beautiful bird of how poor, how unfortunate and how unrewarded they were. The beautiful bird took pity on them, and gave them one shining feather after another, and when it gave its last feather, the bird’s beautiful body was reduced to a sorry state. However the ugly bird did not have any regrets.
The ugly bird that lost its feathers was freezing in the cold, and the child from before appeared in front of it. He told it that he was searching a brightly shining and beautiful bird to repay his debts. The ugly bird was overjoyed, and told the child of its desire. “That was me. Please, would you not keep me warm in your chest?” But the child merely took one glance at the ugly bird and called it a liar, killing it with a huge sword and eating the burnt bird, afterwards he continued to search for the beautiful bird.