Dragon Slayer is an optional extra boss in Sonic and the Black Knight. Not part of the game's main plot, it is found in a Dragon's Lair mission, Special Challenge: Dragon Slayer.
Like the battle with the Mist Dragon, the player also has to slice off the dragon's horn to win. However, this time, the battle is different. The player have to run through a straight pasasage with many enemies and obstacles to reach his horn to get a few cuts. This dragon's horn is apparently stronger than Mist Dragon's as the Mist Dragon's horn can be sliced of by a single cut of a normal sword while this dragon's takes many cuts from Caliburn. After the player reaches the dragon and cuts its horn a few times, the dragon will throw them back to the start of the passage, requiring them to start over. Every time the obstacle course gets harder to beat. After the player reaches the dragon a few times, they will finally manage to cut its horn off, winning the battle and sending the dragon into the deeper parts of the cave.
On some playthroughs of the mission, Sonic makes an oblique reference to Sonic Adventure 2's Finalhazard, mentioning that the last time he fought a giant lizard, it was with Shadow.