The talking sword that accompanies Sonic in Sonic and the Black Knight is Caliburn.
In the game's Arthurian-inspired story, the wizard Merlina summons Sonic to Camelot to oppose King Arthur, whose immortality comes from Excalibur's scabbard. Sonic cannot defeat the corrupted king through speed alone, so he pulls Caliburn from a stone and takes up the role of a knight.
Caliburn is not merely a weapon: he speaks, advises Sonic, and teaches him about chivalry. The sword also helps Sonic find Nimue, the Lady of the Lake, and recover the other sacred swords used by the Round Table knights.
A common mix-up is Excalibur. Excalibur is central to the game's mythology and is Caliburn's ultimate identity after the sacred swords restore him, but the sword's name when Sonic first meets and wields him is Caliburn.